Fix gzeof() to behave just like feof() when read is not past end of file.

Fix gzeof() to behave just like feof() when read is not past end of file.

Before, gzeof() would return true (accurately) when the last read request
went just up to the end of the uncompressed data.  In the analogous case,
feof() would return false, only returning true when a read request goes
past the end of the file.  This patch corrects gzeof() to behave in the
same way as feof(), as noted in the zlib.h documentation.