rhi: Fix memory leak

The leak can be reproduced by running the test project from QTBUG-63557
with asan, although that report is about yet another leak.
[ChangeLog][gui][QRhiGles2] Fixed a memory leak in QRhiGles2

Task-number: QTBUG-63557
Pick-to: 6.1
Change-Id: Ic4d346abb36a5666feb3ceb881865b029f5a6945
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

src/gui/rhi/qrhigles2.cpp

@@ -4163,6 +4163,9 @@ QGles2Buffer::~QGles2Buffer()
 
 void QGles2Buffer::destroy()
 {
+    delete[] data;
+    data = nullptr;
+
     if (!buffer)
         return;
 
@@ -4170,10 +4173,7 @@ void QGles2Buffer::destroy()
     e.type = QRhiGles2::DeferredReleaseEntry::Buffer;
 
     e.buffer.buffer = buffer;
-
     buffer = 0;
-    delete[] data;
-    data = nullptr;
 
     QRHI_RES_RHI(QRhiGles2);
     rhiD->releaseQueue.append(e);
@@ -4197,6 +4197,7 @@ bool QGles2Buffer::create()
             qWarning("Uniform buffer: multiple usages specified, this is not supported by the OpenGL backend");
             return false;
         }
+        delete[] data;
         data = new char[nonZeroSize];
         QRHI_PROF_F(newBuffer(this, uint(nonZeroSize), 0, 1));
         return true;