r466: detect multi-part index more smartly

though it might not work in an extremely rare case: the end of a sequence ends
at X*16384 and it is the last sequence in a batch. This can be resolved by
never letting the kstream_t buffer empty.

minimap2.1

@@ -257,11 +257,6 @@ use
 .TP
 .B --version
 Print version number to stdout
-.TP
-.B --no-sam-hdr
-Don't output SAM header lines. Use this option if the index consists of
-multiple parts; otherwise the SAM output is malformated due to internal header
-lines.
 .SS Preset options
 .TP 10
 .BI -x \ STR