QByteDataBuffer::getChar() - assert that buffer is not empty

Found by CodeChecker.
The getChar() method can return a garbage value if called on an empty
buffer. Considering that QByteDataBuffer is an internal class and that
there seems to be no current usage of this method, just add a Q_ASSERT
to make sure that the buffer is not empty.

Task-number: QTBUG-96303
Pick-to: 6.2
Change-Id: Iab0aee596aabafe999996f83177ca8bba16a58b6
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Ievgenii Meshcheriakov <ievgenii.meshcheriakov@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/text/qbytedata_p.h

@@ -244,6 +244,8 @@ public:
 
     inline char getChar()
     {
+        Q_ASSERT_X(!isEmpty(), "QByteDataBuffer::getChar",
+                   "Cannot read a char from an empty buffer!");
         char c;
         read(&c, 1);
         return c;