fix(inccommand): parse the command to check if it is previewable

Free regprog if command isn't previewable

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

test/functional/ui/inccommand_spec.lua

@@ -3008,3 +3008,16 @@ it('long :%s/ with inccommand does not collapse cmdline', function()
     AAAAAAA^     |
   ]])
 end)
+
+it("with 'inccommand' typing :filter doesn't segfault or leak memory #19057", function()
+  clear()
+  common_setup(nil, 'nosplit')
+  feed(':filter s')
+  assert_alive()
+  feed(' ')
+  assert_alive()
+  feed('h')
+  assert_alive()
+  feed('i')
+  assert_alive()
+end)