Remove usage of CHECK from non-test file

Currently CHECK is used only in test and bench mark files except for one
usage in `ecmult_impl.h`.

We would like to move the definition of CHECK out of `util.h` so that
`util.h` no longer has a hard dependency on `stdio.h`.

Done in preparation for moving the definition of `CHECK` as part of an
effort to allow secp256k1 to be compiled to WASM as part of
`rust-secp256k1`.

Author: tcharding

src/ecmult_impl.h

@@ -200,9 +200,12 @@ static int secp256k1_ecmult_wnaf(int *wnaf, int len, const secp256k1_scalar *a,
         bit += now;
     }
 #ifdef VERIFY
-    CHECK(carry == 0);
-    while (bit < 256) {
-        CHECK(secp256k1_scalar_get_bits(&s, bit++, 1) == 0);
+    VERIFY_CHECK(carry == 0);
+
+    int verify_bit = bit;
+    while (verify_bit < 256) {
+        VERIFY_CHECK(secp256k1_scalar_get_bits(&s, verify_bit, 1) == 0);
+        verify_bit++;
     }
 #endif
     return last_set_bit + 1;