zlib: Fix handling of gzip magic bytes mid-file

Only treat the gzip magic bytes, when encountered within the file
after reading a single block, as the start of a new member when
the previous member has ended.

Add test files that reliably reproduce #5852. The gzipped file
in test/fixtures/pseudo-multimember-gzip.gz contains the gzip
magic bytes exactly at the position that node encounters after having
read a single block, leading it to believe that a new data
member is starting.

Fixes: https://github.com/nodejs/node/issues/5852
PR-URL: https://github.com/nodejs/node/pull/5863


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>