wmadec: Properly handle empty outputs.

wmadec: Properly handle empty outputs.

If out_size is zero we try to shrink the buffer to size zero. POSIX
says that the behavior is implementation-defined in this case, and
para_realloc() aborts due to an assert() statement that checks for
size zero. This patch makes sure the wma decoder never calls realloc()
with a zero size argument.