libpayload/libc: Fix memset/sizeof usage

Since r is a pointer, memset(r, 0, sizeof(r)) would only zero the first
4 (or 8) bytes of the newly allocated struct align_region_t.

An alternative to this patch would be to use calloc, or introduce a new
zalloc (zeroed allocation; a single-element calloc) and use that.

Change-Id: Ic3e3487ce749eeebf6c4836e62b8a305ad766e7e
Found-by: Coverity (ID 1291160)
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/14244
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

payloads/libpayload/libc/malloc.c

@@ -366,7 +366,7 @@ static struct align_region_t *allocate_region(int alignment, int num_elements,
 	if (r == NULL)
 		return NULL;
 
-	memset(r, 0, sizeof(r));
+	memset(r, 0, sizeof(*r));
 
 	if (num_elements != 0) {
 		r->alignment = alignment;