qCompress: return null, not empty, for OOM

While this is a behavior change, we never documented that we'd return
an empty instead of a null QByteArray in this situation. Indeed,
returning bazip, with its fully-preserved capacity, is an expensive
way of signalling an error condition.

Conclude that this is not what the original author wanted and return a
default-constructed byte array instead, leaving bazip and its excess
capacity to be destroyed before the return from the function.

Pick-to: 6.4 6.3 6.2
Task-number: QTBUG-104972
Task-number: QTBUG-106542
Change-Id: I2ee1fc6fdbb9da28f1987b16915516b51bb348c3
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/corelib/text/qbytearray.cpp

@@ -606,9 +606,8 @@ QByteArray qCompress(const uchar* data, qsizetype nbytes, int compressionLevel)
             qToBigEndian(CompressSizeHint_t(nbytes), bazip.data());
             break;
         case Z_MEM_ERROR:
-            qWarning("qCompress: Z_MEM_ERROR: Not enough memory");
-            bazip.resize(0);
-            break;
+            return tooMuchData(ZLibOp::Compression);
+
         case Z_BUF_ERROR:
             len *= 2;
             break;