tools/mxsimage: Remove fclose on empty FILE pointer

If `sb_load_cmdfile()` fails to open the configuration file it will jump
to error handling where the code will try to `fclose()` the FILE pointer
which is NULL causing `mkimage` to segfault.

This patch removes the label for error handling and instead returns
immediately which skips the `fclose()` and prevents the segfault. The
errno is also described in the error message to guide users.

Signed-off-by: Mattias Hansson <hansson.mattias@gmail.com>
Reviewed-by: Wolfgang Denk <wd@denx.de>