Properly initialize an error variable to avoid returning uninitialized

data when 'show drives' succeeds, often resulting in a failing exit code
even though the command worked fine.

Reviewed by:	bz
Approved by:	re (kib)
MFC after:	3 days

usr.sbin/mfiutil/mfi_show.c

@@ -533,6 +533,7 @@ show_drives(int ac, char **av)
 		    MFI_DNAME_ES));
 		printf("\n");
 	}
+	error = 0;
 error:
 	free(list);
 	close(fd);