Switch to our own memcmp function

Fixes #823.

src/util.h

@@ -216,6 +216,24 @@ static SECP256K1_INLINE void memczero(void *s, size_t len, int flag) {
     }
 }
 
+/** Semantics like memcmp. Variable-time.
+ *
+ * We use this to avoid possible compiler bugs with memcmp, e.g.
+ * https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95189
+ */
+static SECP256K1_INLINE int secp256k1_memcmp_var(const void *s1, const void *s2, size_t n) {
+    const unsigned char *p1 = s1, *p2 = s2;
+    size_t i;
+
+    for (i = 0; i < n; i++) {
+        int diff = p1[i] - p2[i];
+        if (diff != 0) {
+            return diff;
+        }
+    }
+    return 0;
+}
+
 /** 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;