Add a few more warnings.

The -Wimplicit-fallthrough one spotted a genuine error too where the
new "samtools split -p" accidentally also enabled --no-PG.

.cirrus.yml

@@ -124,6 +124,7 @@ rocky_task:
     LC_ALL: C
     CIRRUS_CLONE_DEPTH: 1
     HTSDIR: ./hidden-htslib
+    CFLAGS: -Wextra -Wno-sign-compare -Wno-missing-field-initializers -Wno-unused-parameter -Wno-error=old-style-declaration
 
   # NB: we could consider building a docker image with these
   # preinstalled and specifying that instead, to speed up testing.

bam_sort.c

@@ -3079,7 +3079,7 @@ static void *worker(void *data)
             break;
         case MinHash:
             worker_minhash(w);
-            // no break, go to merge sort
+            // fall-through
         default:
             ks_mergesort(sort, w->buf_len, w->buf, 0);
     }

bam_split.c

@@ -171,10 +171,12 @@ static parsed_opts_t* parse_args(int argc, char** argv)
                 return NULL;
             }
             retval->zero_pad = (int) val;
+            break;
         }
         case 1:
             retval->no_pg = 1;
             break;
+
         default:
             if (parse_sam_global_opt(opt, optarg, lopts, &retval->ga) == 0) break;
             /* else fall-through */