fuzz: add missing overrides to signature_checker

and also

- add missing parentheses in fuzz/scriptnum_ops.cpp

- remove useless unsigned int conditional in fuzz/script.cpp

These changes fix 5 compile warnings in gcc 10.

src/test/fuzz/script.cpp

@@ -48,7 +48,7 @@ void test_one_input(const std::vector<uint8_t>& buffer)
     if (CompressScript(script, compressed)) {
         const unsigned int size = compressed[0];
         compressed.erase(compressed.begin());
-        assert(size >= 0 && size <= 5);
+        assert(size <= 5);
         CScript decompressed_script;
         const bool ok = DecompressScript(decompressed_script, size, compressed);
         assert(ok);