Fix two bugs in tabix.c: dead code and missing NULL check

1. Move `detect = 0` outside the error else-block where it was
   unreachable dead code after `return 1`. It now executes when a
   valid -p type is matched, properly suppressing auto-detection.

2. Add a NULL check on the bgzf_open() return value in the is_all
   branch to avoid dereferencing a NULL pointer when the file cannot
   be opened.

Author: sirus20x6

tabix.c

@@ -53,8 +53,8 @@ int main_tabix(int argc, char *argv[])
             else {
                 fprintf(stderr, "The type '%s' not recognised\n", optarg);
                 return 1;
-            detect = 0;
             }
+            detect = 0;
 
         }
     if (optind == argc) {
@@ -77,6 +77,10 @@ int main_tabix(int argc, char *argv[])
         BGZF *fp;
         s.l = s.m = 0; s.s = 0;
         fp = bgzf_open(argv[optind], "r");
+        if (!fp) {
+            fprintf(stderr, "[E::%s] could not open '%s'\n", __func__, argv[optind]);
+            return 1;
+        }
         while (bgzf_getline(fp, '\n', &s) >= 0) puts(s.s);
         bgzf_close(fp);
         free(s.s);