patch 8.2.5063: error for a command may go over the end of IObuff

Problem:    Error for a command may go over the end of IObuff.
Solution:   Truncate the message.

src/ex_docmd.c

@@ -3441,9 +3441,17 @@ theend:
     static void
 append_command(char_u *cmd)
 {
+    size_t  len = STRLEN(IObuff);
     char_u  *s = cmd;
     char_u  *d;
 
+    if (len > IOSIZE - 100)
+    {
+	// Not enough space, truncate and put in "...".
+	d = IObuff + IOSIZE - 100;
+	d -= mb_head_off(IObuff, d);
+	STRCPY(d, "...");
+    }
     STRCAT(IObuff, ": ");
     d = IObuff + STRLEN(IObuff);
     while (*s != NUL && d - IObuff + 5 < IOSIZE)

src/testdir/test_cmdline.vim

@@ -3413,4 +3413,9 @@ func Test_recursive_register()
   call assert_equal('yes', caught)
 endfunc
 
+func Test_long_error_message()
+  " the error should be truncated, not overrun IObuff
+  silent! norm Q00000000000000     000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000                                                                                                                                                                                                                        
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/version.c

@@ -734,6 +734,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    5063,
 /**/
     5062,
 /**/