to return undef when reaching file end

otherwise while(next_aln) loops forever.
to read bl2seq+ output

bl2seq from BLAST+ 2.2.29 uses "Subject=" instead of ">" to start hit lines (ex. Subject= gi|2695846....).
ignore the first group

put in fjossandon's comment.
Bio/SearchIO/blast: Treatment of Subject lines are now consistent
with the treatment of Query lines. Fixed a bug where a
Lamba line without other previous lines where not being parsed
correctly (like in bl2seq outputs). Also added a test case for the
bl2seq BLASTN+ output thanks to yschensandiego.