Check that there is enough space for AvbDescriptor

Test: put corrupted apex on /system_ext/apex
Test: verified it's not activated
Test: atest --host libavb_host_unittest
Bug: 146516087
Change-Id: Ic85cbd580b4a5e6514db68359e78fae7455e0fb8

libavb/avb_descriptor.c

diff --git a/libavb/avb_descriptor.c b/libavb/avb_descriptor.c
index cfc2aac..7030a40 100644
--- a/libavb/avb_descriptor.c
+++ b/libavb/avb_descriptor.c
@@ -88,6 +88,10 @@
   }
 
   for (p = desc_start; p < desc_end;) {
+    if (p + sizeof(AvbDescriptor) > desc_end) {
+      avb_error("Invalid descriptor length.\n");
+      goto out;
+    }
     const AvbDescriptor* dh = (const AvbDescriptor*)p;
     avb_assert_aligned(dh);
     uint64_t nb_following = avb_be64toh(dh->num_bytes_following);