Fix hts_hfile() for compressed text_format files

SAM and VCF files opened for write both start with
htsFile::format.format set to text_format, and then update
it after writing the header.  This means it's possible to open
a text_format file with compression, so hts_hfile() needs to
handle it in the same way as sam and vcf.

Noticed when hts_set_opt(fpout, HTS_OPT_BLOCK_SIZE, BAM_BLOCK_SIZE)
failed in samtools sort in the case where fpout was a compressed
sam file.  At this point fpout was a text_file, so the wrong
pointer was passed to hfile_set_blksize(), which luckily bailed
out and returned -1 before doing any damage to the incorrectly
passed-in structure.

hts.c

@@ -1764,7 +1764,7 @@ static hFILE *hts_hfile(htsFile *fp) {
     case bcf:          // fall through
     case bam:          return bgzf_hfile(fp->fp.bgzf);
     case cram:         return cram_hfile(fp->fp.cram);
-    case text_format:  return fp->fp.hfile;
+    case text_format:  // fall through
     case vcf:          // fall through
     case fastq_format: // fall through
     case fasta_format: // fall through