Memory leak in QSGBatchRenderer::Renderer::map()

In the uncommon case (m_context->hasBrokenIndexBufferObjects() || m_visualizeMode != VisualizeNothing) of mapping a buffer, malloc is called without first freeing the previous malloc.

Attached are the git bisect logs used to track this issue down, a patch to fix this issue, and a testcase to show the memory leak, run it on a system that has Broken Index Buffer Objects and watch the app's memory keep going up.

Regression was introduced with:
qt5 commit: 9347499e78f03710eaf24af3c1e7ac650d0ef81d
qtdeclarative commit: a371bac3fba73f92aaa63a68d8ab1ae81a1d1031