avcodec/utils: initialize pixel buffer pool

This should silence many valgrind warnings about use of uninitialized
data, all the warnings i looked at where false positives having their
uninitialized data not actually used.

The same effect could be achieved by listing all code that touches
the pixel buffer in the valgrind suppression file.
Note, valgrind also fails to trace the origins correctly.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>