mkimage: fit: Free buf directly in fit_extract_data()

If given ptr to free() is NULL, no operation is performed.
Hence we can just free buf directly in fit_extract_data().

Signed-off-by: Bin Meng <bin.meng@windriver.com>
diff --git a/tools/fit_image.c b/tools/fit_image.c
index c37cae3..4aeabbc 100644
--- a/tools/fit_image.c
+++ b/tools/fit_image.c
@@ -534,8 +534,7 @@
 err_munmap:
 	munmap(fdt, sbuf.st_size);
 err:
-	if (buf)
-		free(buf);
+	free(buf);
 	close(fd);
 	return ret;
 }