Fix matching Regex against an empty string

[ErikMinekus]

Matching a Regex against an empty string throws an exception:

Regex regex = new Regex(".*");

int matches = regex.Match("");

Which is caused by this check:

	size_t len = strlen(str);
	if (offset >= len)
	{
		return pCtx->ThrowNativeError("Offset greater or equal than string length\n");
	}

Because offset and len are both 0.

However, this check is redundant, because pcre_exec() already returns PCRE_ERROR_BADOFFSET if the offset is negative or greater than the passed length, and RegexError already supports this.