Bug 1937978 - Exit early with success when decrypting an empty CBCS subsample buffer with ClearKey. r=media-playback-reviewers,aosmond

Without the early exit, DecryptCbcs() will try to get the address of the first
element in the subsample (Span), which with length 0 is forbidden.

This could also be solved by getting the address of the Span through data()
instead, to avoid dereferencing the first element. That seems more like a
footgun than the early exit however.

Differential Revision: https://phabricator.services.mozilla.com/D235310

dom/media/eme/clearkey/ClearKeyUtils.cpp

@@ -109,6 +109,11 @@ bool ClearKeyUtils::DecryptCbcs(const vector<uint8_t>& aKey,
     return false;
   }
 
+  if (aSubsample.Length() == 0) {
+    // Nothing to decrypt.
+    return true;
+  }
+
   std::unique_ptr<PK11SlotInfo, MaybeDeleteHelper<PK11SlotInfo>> slot(
       PK11_GetInternalKeySlot());