'valgrind --tool=exp-sgcheck' detected an invalid write
at former line 491 (sprintf...).

Using snprintf avoids this condition, and allows data loss
detection (Q: why not use the safer c++ strings, A:???).

This fix has some observable impact on bsmap's output,
although I've only seen a change in the amount of whitespace.

I also assume that this might be the cause for the crashes
at the end of bsmap execution.

so that the next commit will look cute :)

Introduce snprintf & LINEBUFFSIZE in similar section for
'single-read alignments'. This should make the code a bit
cleaner.