Add sanity check panic for CRTC interactions

console.c

@@ -144,6 +144,9 @@ cgaputc(int c)
   } else
     crt[pos++] = (c&0xff) | 0x0700;  // black on white
 
+  if(pos > 25*80)
+    panic("pos overflow");
+  
   if((pos/80) >= 24){  // Scroll up.
     memmove(crt, crt+80, sizeof(crt[0])*23*80);
     pos -= 80;