cbfstool: cbfs-mkstage.c: Free `buffer` on error path

Cppcheck warns about a memory leak, present since adding romtool,
which was renamed to cbfstool, in commit 5d01ec0f.

    $ cppcheck --version
    Cppcheck 1.59
    […]
    [cbfs-mkstage.c:170]: (error) Memory leak: buffer
    […]

Indeed the memory pointed to by `buffer` is not freed on the error path,
so add `free(buffer)` to fix this.

Change-Id: I6cbf82479027747c800c5fe847f20b779e261ef4
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/3069
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/cbfstool/cbfs-mkstage.c

@@ -167,6 +167,7 @@ int parse_elf_to_stage(const struct buffer *input, struct buffer *output,
 	if (buffer_create(output, sizeof(*stage) + data_end - data_start,
 			  input->name) != 0) {
 		ERROR("Unable to allocate memory: %m\n");
+		free(buffer);
 		return -1;
 	}
 	memset(output->data, 0, output->size);