Don't use 0 as null pointer constant, part 3

This part replaces E == 0 by !E, where E has pointer type.

src/lib/commands/tran.c

@@ -53,7 +53,7 @@ tran(void)
 
     what = getstarg(player->argp[1], "transport what (nuke or plane): ",
 		    buf);
-    if (what == 0)
+    if (!what)
 	return RET_SYN;
     if (*what == 'n')
 	return tran_nuke();