fix(column): clamp line number for legacy signs

Problem:  Legacy :sign API still allows placing signs beyond the end of
          the buffer. This is unaccounted for by the signcolumn tracking
          logic and is disallowed in general for the extmark API which
          implements it now.
Solution: Clamp legacy sign line number to the length of the buffer.

src/nvim/sign.c

@@ -126,8 +126,8 @@ static void buf_set_sign(buf_T *buf, uint32_t *id, char *group, int prio, linenr
                          | (has_hl ? MT_FLAG_DECOR_SIGNHL : 0);
 
   DecorInline decor = { .ext = true, .data.ext = { .vt = NULL, .sh_idx = decor_put_sh(sign) } };
-  extmark_set(buf, ns, id, lnum - 1, 0, -1, -1, decor, decor_flags, true,
-              false, true, true, NULL);
+  extmark_set(buf, ns, id, MIN(buf->b_ml.ml_line_count, lnum) - 1, 0, -1, -1,
+              decor, decor_flags, true, false, true, true, NULL);
 }
 
 /// For an existing, placed sign with "id", modify the sign, group or priority.

test/functional/ui/sign_spec.lua

@@ -577,4 +577,34 @@ describe('Signs', function()
     ]])
     eq({}, eval('sign_getdefined()'))
   end)
+
+  it('no crash when unplacing signs beyond end of buffer', function()
+    exec([[
+      sign define S1 text=S1
+      sign define S2 text=S2
+      sign place 1 line=8 name=S1
+      sign place 2 line=9 name=S2
+    ]])
+    -- Now placed at end of buffer
+    local s1 = {
+      grid = [[
+        S2^                                                   |
+        {0:~                                                    }|*12
+                                                             |
+      ]],
+    }
+    screen:expect(s1)
+    -- Signcolumn tracking used to not count signs placed beyond end of buffer here
+    exec('set signcolumn=auto:9')
+    screen:expect({
+      grid = [[
+        S2S1^                                                 |
+        {0:~                                                    }|*12
+                                                             |
+      ]],
+    })
+    -- Unplacing the sign does not crash by decrementing tracked signs below zero
+    exec('sign unplace 1')
+    screen:expect(s1)
+  end)
 end)

test/old/testdir/test_signs.vim

@@ -89,8 +89,9 @@ func Test_sign()
   " Place a sign without specifying the filename or buffer
   sign place 77 line=9 name=Sign2
   let a=execute('sign place')
+  " Nvim: sign line clamped to buffer length
   call assert_equal("\n--- Signs ---\nSigns for [NULL]:\n" .
-		\ "    line=9  id=77  name=Sign2  priority=10\n", a)
+		\ "    line=4  id=77  name=Sign2  priority=10\n", a)
   sign unplace *
 
   " Check :jump with file=...
@@ -799,10 +800,11 @@ func Test_sign_group()
   set buftype=nofile
   sign place 25 line=76 name=sign1 priority=99 file=foo
   let a = execute('sign place')
+  " Nvim: sign line clamped to buffer length
   call assert_equal("\n--- Signs ---\nSigns for Xsign:\n" .
 	      \ "    line=10  id=5  name=sign1  priority=10\n" .
 	      \ "Signs for foo:\n" .
-	      \ "    line=76  id=25  name=sign1  priority=99\n", a)
+	      \ "    line=1  id=25  name=sign1  priority=99\n", a)
   close
   bwipe foo