patch 9.1.1851: memory leak in heredoc_get()

Problem:  memory leak in heredoc_get()
Solution: Free "str" if list_append_string() fails
          (Lidong Yan).

In heredoc_get(), eval_all_expr_in_str() allocates a new string and
assign it to local variable "str". When list_append_string() failed
the memory pointed to by "str" leaks. Free "str" if list_append_string()
failed.

related: #18533

Signed-off-by: Lidong Yan <502024330056@smail.nju.edu.cn>
Signed-off-by: Christian Brabandt <cb@256bit.org>

Author: brandb97

src/evalvars.c

@@ -968,7 +968,11 @@ heredoc_get(exarg_T *eap, char_u *cmd, int script_get, int vim9compile)
 	    }
 
 	    if (list_append_string(l, str, -1) == FAIL)
+	    {
+		if (free_str)
+		    vim_free(str);
 		break;
+	    }
 	    if (free_str)
 		vim_free(str);
 	}

src/version.c

@@ -729,6 +729,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1851,
 /**/
     1850,
 /**/