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.
Merge pull request #144 from broadinstitute/broad_bcf_sr_add_reader_segfault_fix

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