Add missing fclose after tape format detection.

Without this, doing something like:
  devmount mta0 /tmp/foo
  devunmount mta0
(i.e. without specifying fmt=bar to the devmount) will leak a FILE * and
file descriptor.

src/vmtape.c

@@ -886,6 +886,8 @@ vmt_rdmount(register struct vmtape *t,
 	    return FALSE;
 	}
 	dfn = ta->vmta_path;
+	fclose(df);
+	df = NULL;
     }
  havefmt:
     if (vmtfmttab[fmt].tf_flags & (VMTFF_CTL | VMTFF_XCTL)) {