Fix mg_get_http_var retval

PUBLISHED_FROM=257deff5ea20302627e77a0f29cf2359276a41d4

mongoose.h

@@ -4516,7 +4516,8 @@ size_t mg_parse_multipart(const char *buf, size_t buf_len, char *var_name,
  * Fetches a variable `name` from a `buf` into a buffer specified by `dst`,
  * `dst_len`. The destination is always zero-terminated. Returns the length of
  * a fetched variable. If not found, 0 is returned. `buf` must be valid
- * url-encoded buffer. If destination is too small, `-1` is returned.
+ * url-encoded buffer. If destination is too small or an error occured,
+ * negative number is returned.
  */
 int mg_get_http_var(const struct mg_str *buf, const char *name, char *dst,
                     size_t dst_len);