Fix buffer overflow in guru chat parsing engine

The trigger was this line in the recent change to guru.dat:
Unrealized potential. :-(

That open paren was parsed as the beginning of a new guru expression. A very
long guru expression that overflowed the stack variable 'str' here.

src/sbbs3/chat.cpp

@@ -1826,7 +1826,8 @@ bool sbbs_t::guruexp(char **ptrptr, char *line)
 		while((**ptrptr) && IS_WHITESPACE(**ptrptr))
 			(*ptrptr)++;
 		while((**ptrptr)!='|' && (**ptrptr)!='&' && (**ptrptr)!=')' &&(**ptrptr)) {
-			str[c++]=(**ptrptr);
+			if(c < sizeof(str) - 1)
+				str[c++]=(**ptrptr);
 			(*ptrptr)++; 
 		}
 		str[c]=0;