QByteDataBuffer: assert an undocumented precondition

The alternative would be to return a null QByteArray, but the current
implementation makes calling read() on an empty QByteDataBuffer UB, so
this is behavior-preserving, just with a bit nicer error message.

Pick-to: 6.4 6.3 6.2
Change-Id: Ia9db7acbfa47e601b466d6cc6a728859065c27b5
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/corelib/text/qbytedata_p.h

@@ -105,6 +105,7 @@ public:
     // preferably use this function to read data.
     inline QByteArray read()
     {
+        Q_ASSERT(!isEmpty());
         squeezeFirst();
         bufferCompleteSize -= buffers.first().size();
         return buffers.takeFirst();