Bug 1650281 - P2: Make sure `gCombinedSizes` won't be underflowed r=gerald

In any case, `gCombinedSizes` should be larger than or equal to the
buffer within `MemoryClockCache`.

Differential Revision: https://phabricator.services.mozilla.com/D84274

dom/media/MemoryBlockCache.cpp

@@ -52,6 +52,7 @@ MemoryBlockCache::MemoryBlockCache(int64_t aContentLength)
 }
 
 MemoryBlockCache::~MemoryBlockCache() {
+  MOZ_ASSERT(gCombinedSizes >= mBuffer.Length());
   size_t sizes = static_cast<size_t>(gCombinedSizes -= mBuffer.Length());
   LOG("~MemoryBlockCache() - destroying buffer of size %zu; combined sizes now "
       "%zu",