Also treat file reading as an error if ftell reports invalid length.

`ftell` is documented to return -1L if the function failed to get the current position indicator in the stream.  The possibility is easily set aside just by checking if the function returned a negative length.

file_ops.c

@@ -110,6 +110,8 @@ static bool read_generic_file(const char *path, void **buf, ssize_t *len)
    fseek(file, 0, SEEK_END);
 
    _len     = ftell(file);
+   if (_len < 0)
+      goto error;
 
    rewind(file);