Fix crash caused by stupid people doing stupid things.

Now when you can't allocate the second GB of RAM to show your billion line
scrollback, will simply not show you the 1GB of scrollback you have amassed.

You still won't be able to see it, but it won't crash anymore.

Reported by Digital Man.

src/syncterm/menu.c

@@ -31,6 +31,8 @@ void viewscroll(void)
     gettextinfo(&txtinfo);
 	/* too large for alloca() */
 	scrollback=(char *)malloc((scrollback_buf==NULL?0:(term.width*2*settings.backlines))+(txtinfo.screenheight*txtinfo.screenwidth*2));
+	if(scrollback==NULL)
+		return;
 	if(cterm->scrollback != NULL)
 		memcpy(scrollback,cterm->scrollback,term.width*2*settings.backlines);
 	gettext(1,1,txtinfo.screenwidth,txtinfo.screenheight,scrollback+(cterm->backpos)*cterm->width*2);