Fix hungry-matching of "NODEx" @-code

The 5th character must be a digit.

Prevent future stupidity such as happened with recently added and then
fixed NODE_USER @-code.

src/sbbs3/atcodes.cpp

@@ -941,7 +941,7 @@ const char* sbbs_t::atcode(const char* sp, char* str, size_t maxlen, int* pmode,
 	if(strcmp(sp, "NODE_USER") == 0)
 		return thisnode.misc&NODE_ANON ? text[UNKNOWN_USER] : useron.alias;
 
-	if(!strncmp(sp,"NODE",4)) {
+	if(!strncmp(sp,"NODE",4) && IS_DIGIT(sp[4])) {
 		i=atoi(sp+4);
 		if(i && i<=cfg.sys_nodes) {
 			getnodedat(i,&node,0);