LZX, XPRESS decompression: Return 0 bits on overrun

LZX, XPRESS decompression:  Return 0 bits on overrun

If the compressed data is invalid such that the compressed data buffer is
overrun, it's simpler to just return 0 bits instead of explicitly
checking the return value at every call site of bitstream_read_bits() and
read_huffsym().

This doesn't necessarily mean that invalid data will go undetected.  Just
for LZX decompression, chances are there will be another problem if all
0's start being returned (e.g. invalid match or invalid Huffman tree).
For WIM operations like extraction, the uncompressed data is checked with
SHA-1 message digests anyway, so it's virtually impossible for corruption
to go undetected.

Also, the LZMS decompressor already does this.