Ensure NULL is not returned incorrectly after the first call.

I'm surprised this was not caught long ago, but previous the
code actually never did call the function twice so it was
missed.  Anyway, this fixes it.

lib/libspl/getexecname.c

@@ -48,6 +48,8 @@ getexecname(void)
 			execname[rc] = '\0';
 			ptr = execname;
 		}
+	} else {
+		ptr = execname;
 	}
 
 	pthread_mutex_unlock(&mtx);