json_load_file: Initialize the error struct properly

Failing to do this has the effect that the error message is not
returned when the input file cannot be opened (e.g. if it doesn't
exist).

Thanks to Martin Vopatek for reporting.

src/load.c

@@ -864,6 +864,8 @@ json_t *json_load_file(const char *path, json_error_t *error)
     json_t *result;
     FILE *fp;
 
+    error_init(error);
+
     fp = fopen(path, "r");
     if(!fp)
     {