util: after gzgets(), Z_STREAM_END means EOF, not error

Context: I am compiling notmuch on OpenBSD which has a rather old zlib
1.2.3.  It seems that the behaviour of gzgets() changed slightly between
this version and more recent versions, but the manual does not reflect
that change.  Note that zlib's manual:
- does not specify which error code (Z_OK or Z_STREAM_END) is set when
  EOF is reached,
- does not indicate the meaning of Z_STREAM_END after gzgets(), but
  based on its meaning as a possible return value of inflate(), I would
  guess that it means EOF.

amended by db: tidy commit message