Fix CID 508288 (STRING_NULL)

I'm not clear why Coverity suddenly thinks that getstr() doesn't
nul-terminate strings, but whatever: NUL-terminate the buf before passing it
to getstr(). <shrug>

src/sbbs3/telgate.cpp

@@ -381,6 +381,7 @@ bool sbbs_t::telnet_gate(char* destaddr, uint mode, unsigned timeout, str_list_t
 					l=K_CHAT;
 					if(!(mode&TG_ECHO))
 						l|=K_NOECHO;
+					*buf = '\0';
 					rd=getstr((char*)buf,sizeof(buf)-1,l);
 					if(!rd)
 						continue;