bcf_sr_add_reader(): do not increment nreaders or perform any reallocs upon file open errors

This avoids a segfault if bcf_sr_destroy() is called after bcf_sr_add_reader()
when a file couldn't be opened.

NOTE: will not apply cleanly to upstream -- we'll need to deal with the conflict
during rebase
Merge pull request #6 from broadinstitute/dr_fix_synced_bcf_reader_segfault

bcf_sr_add_reader(): do not increment nreaders or perform any reallocs upon file open errors