util: add constant-time is_zero_array function

src/util.h

@@ -239,6 +239,22 @@ static SECP256K1_INLINE int secp256k1_memcmp_var(const void *s1, const void *s2,
     return 0;
 }
 
+/* Return 1 if all elements of array s are 0 and otherwise return 0.
+ * Constant-time. */
+static SECP256K1_INLINE int secp256k1_is_zero_array(const unsigned char *s, size_t len) {
+    unsigned char acc = 0;
+    int ret;
+    size_t i;
+
+    for (i = 0; i < len; i++) {
+        acc |= s[i];
+    }
+    ret = (acc == 0);
+    /* acc may contain secret values. Try to explicitly clear it. */
+    acc = 0;
+    return ret;
+}
+
 /** If flag is true, set *r equal to *a; otherwise leave it. Constant-time.  Both *r and *a must be initialized and non-negative.*/
 static SECP256K1_INLINE void secp256k1_int_cmov(int *r, const int *a, int flag) {
     unsigned int mask0, mask1, r_masked, a_masked;