In OpenGLSalBitmap::AcquireBuffer(), if ReadTexture() failed, then
the data from AllocateUserData() didn't get deallocated and a next
call to OpenGLSalBitmap::AcquireBuffer() skipped the whole block
because it assumed the data was valid. Triggered while fixing tdf#116888.

Change-Id: Ibfe5c42d6b18748ca649d6b4242ef268c1b13a71
Reviewed-on: https://gerrit.libreoffice.org/69746
Tested-by: Jenkins
Reviewed-by: Tomaž Vajngerl <quikee@gmail.com>