str_format_meta: missing initializer on error path

(cherry picked from commit b592e72f5c2a7f7798ca91b3ce46e19257bd9a36)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

src/text/strings.c

@@ -818,6 +818,8 @@ char *str_format_meta(input_thread_t *input, const char *s)
         fread(str, len, 1, stream);
         str[len] = '\0';
     }
+    else
+        str = NULL;
     fclose(stream);
     return str;
 #endif