Fix lost 'unexpected characters' received in ANSI get cursor position response

Unexpected characters received (maybe typed by the user) were supposed to
be stuffed in the keyboard input buffer using ungetstr(), but the input string
was cleared rather than NUL-terminated before logging the expected character
and calling ungetstr(), so nothing (a blank string) was logged and nothing was
stuffed in the keyboard input-buffer in this case. Just an off-by-one bug.

This should fix issue #304. Thanks to Keyop and Nelgin for their persistence
in reporting and testing (in #synchronet at irc.synchro.net).

src/sbbs3/ansiterm.cpp

@@ -250,7 +250,7 @@ bool sbbs_t::ansi_getxy(int* x, int* y)
             else if(ch=='R' && rsp)
             	break;
 			else {
-				str[rsp] = 0;
+				str[rsp + 1] = 0;
 #ifdef _DEBUG
 				char dbg[128];
 				c_escape_str(str, dbg, sizeof(dbg), /* Ctrl-only? */true);