Fix error check in nvlist_print_json_string

Move check for errors from mbrtowc() into the loop.  The error values
are not actually negative, so we don't break out of the loop when they
are encountered.

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes openzfs#12175
Closes openzfs#12176

lib/libnvpair/libnvpair_json.c

@@ -54,6 +54,13 @@ nvlist_print_json_string(FILE *fp, const char *input)
 
 	FPRINTF(fp, "\"");
 	while ((sz = mbrtowc(&c, input, MB_CUR_MAX, &mbr)) > 0) {
+		if (sz == (size_t)-1 || sz == (size_t)-2) {
+			/*
+			 * We last read an invalid multibyte character sequence,
+			 * so return an error.
+			 */
+			return (-1);
+		}
 		switch (c) {
 		case '"':
 			FPRINTF(fp, "\\\"");
@@ -97,14 +104,6 @@ nvlist_print_json_string(FILE *fp, const char *input)
 		input += sz;
 	}
 
-	if (sz == (size_t)-1 || sz == (size_t)-2) {
-		/*
-		 * We last read an invalid multibyte character sequence,
-		 * so return an error.
-		 */
-		return (-1);
-	}
-
 	FPRINTF(fp, "\"");
 	return (0);
 }