patch 8.2.3850: illegal memory access when displaying a partial

Problem:    Illegal memory access when displaying a partial.
Solution:   Terminate the string with a NUL. (closes #9371)

src/eval.c

@@ -5087,7 +5087,8 @@ echo_string_core(
 		    ga_concat(&ga, tv2string(&dtv, &tf, numbuf, copyID));
 		    vim_free(tf);
 		}
-		ga_concat(&ga, (char_u *)")");
+		// terminate with ')' and a NUL
+		ga_concat_len(&ga, (char_u *)")", 2);
 
 		*tofree = ga.ga_data;
 		r = *tofree;

src/testdir/test_messages.vim

@@ -337,4 +337,11 @@ func Test_quit_long_message()
   call delete('Xtest_quit_message')
 endfunc
 
+" this was missing a terminating NUL
+func Test_echo_string_partial()
+  function CountSpaces()
+  endfunction
+  echomsg function('CountSpaces', [#{aaaaaaaaaaa: v:false, bbbbbbbbbbbb: '', ccccccccccc: ['ab', 'cd']}])
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -749,6 +749,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3850,
 /**/
     3849,
 /**/