put_bits: fix invalid shift by 32 in flush_put_bits()

If flush_put_bits() is called when the 32-bit buffer is empty,
e.g. after writing a multiple of 32 bits, and invalid shift by
32 is performed.  Since flush_put_bits() is called infrequently,
this additional check should have negligible performance impact.

Signed-off-by: Mans Rullgard <mans@mansr.com>
