Fix ks_getuntil2() extra empty record at EOF bug

Fix ks_getuntil2() extra empty record at EOF bug

When the stream is an exact multiple of the buffer size, ks_getuntil2()
was returning a final empty record when it should have returned -1.
Fixed by moving the "EOF => return -1" check to after the read loop.

(See samtools/samtools#318 for an example of an error caused by a
spurious empty line at the end of a SAM file.)