t/test-gh1.sh: discard stdout from mtbl_dump
reader: get_block(): use snappy_uncompressed_length() to get the real value of block_contents_size

this addresses the MTBL_COMPRESSION_SNAPPY case for Issue #1.
reader: factor out zlib decompression into get_block_zlib_decompress()
reader: get_block_zlib_decompress(): handle Z_BUF_ERROR from inflate()

for highly compressed data, the output buffer we pass to zlib's
inflate() may not be large enough to contain the decompressed data
block. call inflate() in a loop, enlarging the output buffer as
necessary.

this addresses the MTBL_COMPRESSION_ZLIB case for Issue #1.
reader: get_block(): increase initial zlib decompression buffer size

after the previous commit, we now correctly handle the case where the
decompression buffer is (initially) too small to hold the decompressed
output. however, we can still increase the initial size of this buffer
in order to reduce the amount of looping and reallocation that has to be
done in that case. so, increase it from 2X to 4X of the data block size.