core: fix possible overflow in shdr_alloc_and_copy()

Prior to this patch, if SHDR_GET_SIZE() overflows it will return 0 and
further down in the function lead to an out-of-bounds access. So fix
this with an explicit test before using shdr_size in
shdr_alloc_and_copy().

Fixes: 064663e ("core: crypto: add struct shdr helper functions")
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>

core/crypto/signed_hdr.c

@@ -29,7 +29,7 @@ struct shdr *shdr_alloc_and_copy(size_t offs, const void *img, size_t img_size)
 		return NULL;
 
 	shdr_size = SHDR_GET_SIZE((const struct shdr *)(img_va + offs));
-	if (ADD_OVERFLOW(offs, shdr_size, &end) || end > img_size)
+	if (!shdr_size || ADD_OVERFLOW(offs, shdr_size, &end) || end > img_size)
 		return NULL;
 
 	if (ADD_OVERFLOW(img_va, shdr_size, &tmp))