eval: let getreg() return valid list for an undefined register

vim-patch:7.4.1755
this prevents a crash when the list is used in setreg() later

src/nvim/eval.c

@@ -10096,9 +10096,10 @@ static void f_getreg(typval_T *argvars, typval_T *rettv)
     rettv->v_type = VAR_LIST;
     rettv->vval.v_list = 
       get_reg_contents(regname, (arg2 ? kGRegExprSrc : 0) | kGRegList);
-    if (rettv->vval.v_list != NULL) {
-      rettv->vval.v_list->lv_refcount++;
+    if (rettv->vval.v_list == NULL) {
+      rettv->vval.v_list = list_alloc();
     }
+    rettv->vval.v_list->lv_refcount++;
   } else {
     rettv->v_type = VAR_STRING;
     rettv->vval.v_string = get_reg_contents(regname, arg2 ? kGRegExprSrc : 0);

src/nvim/version.c

@@ -69,6 +69,7 @@ static char *features[] = {
 
 // clang-format off
 static int included_patches[] = {
+  1755,
   1654,
   1652,
   1643,

test/functional/legacy/eval_spec.lua

@@ -495,6 +495,28 @@ describe('eval', function()
       ' abcE4b10-4\000abcE4b10-4-2')
   end)
 
+  it('getreg("a",1,1) returns a valid list when "a is unset', function()
+    -- Precondition: "a is actually unset and "0 is nonempty
+    eq('', eval("getregtype('a')"))
+    eq('', eval("getreg('a')"))
+    execute("call setreg('0','text')")
+
+    -- This used to return a NULL list
+    -- which setreg didn't handle
+    execute("let x = getreg('a',1,1)")
+    execute("call setreg('0',x)")
+
+    -- nvim didn't crash and "0 was emptied
+    eq(2, eval("1+1"))
+    eq({}, eval("getreg('0',1,1)"))
+
+    -- x is a mutable list
+    execute("let y = x")
+    eq({}, eval("y"))
+    execute("call add(x, 'item')")
+    eq({'item'}, eval("y"))
+  end)
+
   it('search and expressions', function()
     execute('so test_eval_setup.vim')
     execute([=[call SetReg('/', ['abc/'])]=])

test/functional/shada/compatibility_spec.lua

@@ -270,9 +270,7 @@ describe('ShaDa forward compatibility support code', function()
   it('works with register item with type 10', function()
     wshada('\005\001\019\132\161na\162rX\194\162rc\145\196\001-\162rt\010')
     eq(0, exc_exec(sdrcmd(true)))
-    -- getreg may return empty list as list with NULL pointer which API 
-    -- translates into nil for some reason.
-    eq(NIL, funcs.getreg('a', 1, 1) or {})
+    eq({}, funcs.getreg('a', 1, 1))
     eq('', funcs.getregtype('a'))
     nvim_command('wshada ' .. shada_fname)
     local found = 0

test/functional/shada/registers_spec.lua

@@ -43,9 +43,9 @@ describe('ShaDa support code', function()
     setreg('b', {'bca', 'abc', 'cba'}, 'b3')
     nvim_command('qall')
     reset()
-    eq({NIL, ''}, getreg('c'))
-    eq({NIL, ''}, getreg('l'))
-    eq({NIL, ''}, getreg('b'))
+    eq({{}, ''}, getreg('c'))
+    eq({{}, ''}, getreg('l'))
+    eq({{}, ''}, getreg('b'))
   end)
 
   it('does restore registers with zero <', function()
@@ -67,9 +67,9 @@ describe('ShaDa support code', function()
     setreg('b', {'bca', 'abc', 'cba'}, 'b3')
     nvim_command('qall')
     reset()
-    eq({NIL, ''}, getreg('c'))
-    eq({NIL, ''}, getreg('l'))
-    eq({NIL, ''}, getreg('b'))
+    eq({{}, ''}, getreg('c'))
+    eq({{}, ''}, getreg('l'))
+    eq({{}, ''}, getreg('b'))
   end)
 
   it('does restore registers with zero "', function()
@@ -103,7 +103,7 @@ describe('ShaDa support code', function()
     nvim_command('qall')
     reset()
     eq({{'d'}, 'v'}, getreg('o'))
-    eq({NIL, ''}, getreg('t'))
+    eq({{}, ''}, getreg('t'))
   end)
 
   it('does limit number of lines according to "', function()
@@ -113,7 +113,7 @@ describe('ShaDa support code', function()
     nvim_command('qall')
     reset()
     eq({{'d'}, 'v'}, getreg('o'))
-    eq({NIL, ''}, getreg('t'))
+    eq({{}, ''}, getreg('t'))
   end)
 
   it('does limit number of lines according to < rather then "', function()
@@ -125,7 +125,7 @@ describe('ShaDa support code', function()
     reset()
     eq({{'d'}, 'v'}, getreg('o'))
     eq({{'a', 'b', 'cde'}, 'V'}, getreg('t'))
-    eq({NIL, ''}, getreg('h'))
+    eq({{}, ''}, getreg('h'))
   end)
 
   it('dumps and loads register correctly when &encoding is not UTF-8',