Exit with error if a new buffer can't be allocated

When building with gcc and -Wnull-dereference, -O3 and -flto we get a
lot of warnings about potential null dereferences. Exiting instead of
returning NULL solves all of it.

If we cannot alloc memory, there is not much to do - exiting is the
easiest option.

buffer.c

@@ -28,7 +28,7 @@ newBuffer(int width)
 
     n = New(Buffer);
     if (n == NULL)
-	return NULL;
+	exit(3);
     bzero((void *)n, sizeof(Buffer));
     n->width = width;
     n->COLS = COLS;