Fix off-by-one number of extra operations

This caused a performance issue.
diff --git a/tinycrypt/ecc.c b/tinycrypt/ecc.c
index 6740375..d01c676 100644
--- a/tinycrypt/ecc.c
+++ b/tinycrypt/ecc.c
@@ -342,7 +342,7 @@
 	delays >>= 2;
 	/* k = 0 -> i in [1, 0] -> 0 extra muladd;
 	 * k = 3 -> i in [1, 3] -> 3 extra muladd */
-	for (i = 0; i <= k; ++i) {
+	for (i = 1; i <= k; ++i) {
 		muladd(left[i], right[k - i], &rr0, &rr1, &r2);
 	}
 	r = rr0;