Separate secp256k1_fe_set_int( . , 0 ) from secp256k1_fe_clear()

There are two uses of the secp256k1_fe_clear() function that are now separated
into these two functions in order to reflect the intent:

1) initializing the memory prior to being used -> converted to fe_set_int( . , 0 )
2) zeroing the memory after being used such that no sensitive data remains. ->
    remains as fe_clear()

In the latter case, 'magnitude' and 'normalized' need to be overwritten when
VERIFY is enabled.

Co-Authored-By: isle2983 <isle2983@yahoo.com>

src/testutil.h

@@ -34,7 +34,7 @@ static void testutil_random_fe_magnitude(secp256k1_fe *fe, int m) {
     if (n == 0) {
         return;
     }
-    secp256k1_fe_clear(&zero);
+    secp256k1_fe_set_int(&zero, 0);
     secp256k1_fe_negate(&zero, &zero, 0);
     secp256k1_fe_mul_int_unchecked(&zero, n - 1);
     secp256k1_fe_add(fe, &zero);