[lldb] Don't rely on wrapping in PutRawBytes/PutBytesAsRawHex8

I was looking at Stream::PutRawBytes and thought I spotted a bug because
both loops are using `i < src_len` as the loop condition despite them
iterating in opposite directions.

On closer inspection, the existing code is correct, because it relies on
well-defined unsigned integer wrapping. Correct doesn't mean readable,
so this patch changes the loop condition to compare against 0 when
decrementing i while still covering the edge case of src_len potentially
being 0 itself.

Differential revision: https://reviews.llvm.org/D119857