Fix crash in enline

demo'ed by this code:
>> enline "^/a^/b"

This fixes zsx#16

Author: zsx

src/core/s-ops.c

@@ -458,10 +458,11 @@ static REBYTE seed_str[SEED_LEN] = {
 	// Add missing CRs:
 	while (cnt > 0) {
 		bp[tail--] = bp[len]; // Copy src to dst.
-		if (bp[len--] == LF && bp[len] != CR) {
+		if (bp[len] == LF && (len == 0 || bp[len - 1] != CR)) {
 			bp[tail--] = CR;
 			cnt--;
 		}
+		len--;
 	}
 }
 
@@ -495,10 +496,11 @@ static REBYTE seed_str[SEED_LEN] = {
 	// Add missing CRs:
 	while (cnt > 0) {
 		bp[tail--] = bp[len]; // Copy src to dst.
-		if (bp[len--] == LF && bp[len] != CR) {
+		if (bp[len] == LF && (len == 0 || bp[len] != CR)) {
 			bp[tail--] = CR;
 			cnt--;
 		}
+		len--;
 	}
 }