Security: Static/Dynamic string compare functions aren't properly handling empty...

Security: Static/Dynamic string compare functions aren't properly handling empty strings or empty ranges.

When the static/dynamic string has used = 0 or the range.start > range.stop, then the string (or range) is empty.
The private function isn't aware of this and simply attempts to access the invalid ranges.
This will result in an invalid read.

Add the missing used and range checks.