refactor: Make 64-bit shift explicit

This change fixes MSVC level-3 warning C4334.
See: https://learn.microsoft.com/en-us/cpp/error-messages/compiler-warnings/compiler-warning-level-3-c4334

Required to enable level 3 warnings (/W3).

src/ecmult_impl.h

@@ -683,7 +683,7 @@ static int secp256k1_ecmult_pippenger_batch(const secp256k1_callback* error_call
     }
     state_space->ps = (struct secp256k1_pippenger_point_state *) secp256k1_scratch_alloc(error_callback, scratch, entries * sizeof(*state_space->ps));
     state_space->wnaf_na = (int *) secp256k1_scratch_alloc(error_callback, scratch, entries*(WNAF_SIZE(bucket_window+1)) * sizeof(int));
-    buckets = (secp256k1_gej *) secp256k1_scratch_alloc(error_callback, scratch, (1<<bucket_window) * sizeof(*buckets));
+    buckets = (secp256k1_gej *) secp256k1_scratch_alloc(error_callback, scratch, ((size_t)1 << bucket_window) * sizeof(*buckets));
     if (state_space->ps == NULL || state_space->wnaf_na == NULL || buckets == NULL) {
         secp256k1_scratch_apply_checkpoint(error_callback, scratch, scratch_checkpoint);
         return 0;

src/tests.c

@@ -2221,7 +2221,7 @@ static void scalar_test(void) {
         for (i = 0; i < 100; ++i) {
             int low;
             int shift = 1 + secp256k1_testrand_int(15);
-            int expected = r.d[0] % (1 << shift);
+            int expected = r.d[0] % (1ULL << shift);
             low = secp256k1_scalar_shr_int(&r, shift);
             CHECK(expected == low);
         }