Fix heap-buffer-overflow in raw file loading

When `jv_string_append_buf` overflows the string length limit,
it returns an invalid `jv`; `jv_load_file` then re-entered it
on the invalid value and overran the heap. Break out of the loop
once the value is invalid.

Fixes CVE-2026-49839.

src/jv_file.c

@@ -57,6 +57,8 @@ jv jv_load_file(const char* filename, int raw) {
 
     if (raw) {
       data = jv_string_append_buf(data, buf, n);
+      if (!jv_is_valid(data))
+        break;
     } else {
       jv_parser_set_buf(parser, buf, n, !feof(file));
       jv value;