draw: avoid overflows in the llvm draw loop

draw: avoid overflows in the llvm draw loop

Before we could easily overflow if start+count>max integer. To
avoid it we can just iterate over the count. This makes sure
that we never crash, since most of the overflow conditions
is already handled.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>