Fix style in execsh

st.c

@@ -1329,9 +1329,8 @@ execsh(void)
 			die("who are you?\n");
 	}
 
-	if (!(sh = getenv("SHELL"))) {
+	if ((sh = getenv("SHELL")) == NULL)
 		sh = (pw->pw_shell[0]) ? pw->pw_shell : shell;
-	}
 
 	if (opt_cmd)
 		prog = opt_cmd[0];