glbufferpool: explicitly initialize a possibly uninitialized variable

glbufferpool: explicitly initialize a possibly uninitialized variable

The ret variable may be uninitialized and so its contents were undefined and
the results were erratic (failing with glvideomixer, succeeding in other cases)

P.S. No idea why gcc/clang et al never picked up on this like they normally do
(probably due to some optimisation pass figuring out it's only set once...)