Bug 1675465 - Add DIAGNOSTIC_ASSERT in nsJARInputStream::Read r=jstutte

We want to make sure we don't accidentally read more than the size of the file.

Differential Revision: https://phabricator.services.mozilla.com/D96006

modules/libjar/nsJARInputStream.cpp

@@ -220,6 +220,8 @@ nsJARInputStream::Read(char* aBuffer, uint32_t aCount, uint32_t* aBytesRead) {
 
     case MODE_COPY:
       if (mFd) {
+        MOZ_DIAGNOSTIC_ASSERT(mOutSize >= mZs.total_out,
+                              "Did we read more than expected?");
         uint32_t count = std::min(aCount, mOutSize - uint32_t(mZs.total_out));
         if (count) {
           memcpy(aBuffer, mZs.next_in + mZs.total_out, count);