If the decompression stream was terminated with a 1 bit at exactly the
byte boundary (perhaps due to decompressing from flash memory and
sinking erased 0xFF bytes until the end of a flash page), it could end
up in a state where heatshrink_decoder_finish() would return
HSDR_FINISH_MORE, but calling heatshrink_decoder_poll would always yield
0 bytes of output. If _both_ results were not checked, this could lead to
the code driving them uselessly calling them in a loop.

Added regression test case:
decoder_should_not_get_stuck_with_finish_yielding_MORE_but_0_bytes_output_from_poll

Resolved by explicitly checking remaining input length in
heatshrink_decoder_finish in the HSDS_YIELD_LITERAL state, since it is
not safe to assume that the decompression stream input will be padded
with 0 bits.