Fix FindStringIndex native not returning INVALID_STRING_INDEX when string not found (bug 6144).

This should resolve bug 6144. The SP definition of INVALID_STRING_INDEX in sdktools_stringtables.inc doesn't match the sdk version. (-1 and 65535 respectively).

Updating the define in the SP inc file to be "correct" would cause plugins to need to be recompiled to get the fix. Instead, I've updated the only native that uses this to return the value that plugins are expecting.