libpayload: free: Separate NULL check out for clarity

Separate the validity check of calling free(NULL) for clarity.

BUG=b:168441735
TEST=emerge-puff libpayload

Change-Id: I0dc355553410bbe59e658945fb40c05f5f709380
Signed-off-by: Hsuan Ting Chen <roccochen@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/45465
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Yu-Ping Wu <yupingso@google.com>

payloads/libpayload/libc/malloc.c

@@ -246,6 +246,10 @@ void free(void *ptr)
 	hdrtype_t hdr;
 	struct memory_type *type = heap;
 
+	/* No action occurs on NULL. */
+	if (ptr == NULL)
+		return;
+
 	/* Sanity check. */
 	if (ptr < type->start || ptr >= type->end) {
 		type = dma;