Fix a crash on 64-bit systems

When index = tail, it will crash on 64-bit systems:

REBCNT out = 0 here:
for (out--; out >= start; out--) {
	//out = 0xFFFFFFFF (or -1 as a signed integer)
	uc = GET_ANY_CHAR(ser, out); //expands to ser->data[out], which
is the same as *(ser->data + out). On 32-bit system, ser->data is
32-bit, so it's equivalent to *(ser->data - 1), which is a valid address,
but wrong content; while on 64-bit system, ser->data is 64-bit, so "out"
has to be promoted to 64-bit integer, which results in *(ser->data +
0xFFFFFFFF), which is not a valid memory location.
    ...
}

src/core/s-trim.c

@@ -206,7 +206,9 @@ static REBFLG find_in_uni(REBUNI *up, REBINT len, REBUNI c)
 	REBCNT out = index;
 	REBOOL append_line_feed = FALSE;
 	REBUNI uc;
-
+	if (tail == index){
+		return;
+	}
 	// Skip head lines if required:
 	if (h || !t) {
 		for (; index < tail; index++) {