HPack: fix incorrect integer overflow check

This code never worked:

For the comparison with max() - 32 to trigger, on 32-bit platforms (or
Qt 5) signed interger overflow would have had to happen in the
addition of the two sizes. The compiler can therefore remove the
overflow check as dead code.

On Qt 6 and 64-bit platforms, the signed integer addition would be
very unlikely to overflow, but the following truncation to uint32
would yield the correct result only in a narrow 32-value window just
below UINT_MAX, if even that.

Fix by using the proper tool, qAddOverflow.

Pick-to: 6.7 6.6 6.5 6.2 5.15
Change-Id: I7599f2e75ff7f488077b0c60b81022591005661c
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/network/access/http2/hpacktable.cpp

@@ -26,7 +26,9 @@ HeaderSize entry_size(QByteArrayView name, QByteArrayView value)
     // for counting the number of references to the name and value would have
     // 32 octets of overhead."
 
-    const unsigned sum = unsigned(name.size() + value.size());
+    size_t sum;
+    if (qAddOverflow(size_t(name.size()), size_t(value.size()), &sum))
+        return HeaderSize();
     if (sum > (std::numeric_limits<unsigned>::max() - 32))
         return HeaderSize();
     return HeaderSize(true, quint32(sum + 32));