Fix compilation with optimization with g++-10.2

When compiling in RelWithDebInfo configuration with g++-10.2,
it complains:
qtbase/src/gui/painting/qdatabuffer_p.h:71:36: error: argument 1 range
[18446744065119617024, 18446744073709551612] exceeds maximum object size
9223372036854775807 [-Werror=alloc-size-larger-than=]
This patch disables this warning in this place.

Pick-to: 5.15
Change-Id: Id3154ce80a1921671031828aaa7144214adad5dd
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/gui/painting/qdatabuffer_p.h

@@ -66,7 +66,10 @@ public:
     {
         capacity = res;
         if (res) {
+            QT_WARNING_PUSH
+            QT_WARNING_DISABLE_GCC("-Walloc-size-larger-than=")
             buffer = (Type*) malloc(capacity * sizeof(Type));
+            QT_WARNING_POP
             Q_CHECK_PTR(buffer);
         } else {
             buffer = nullptr;