Fix miscellaneous style nits that irritate overactive static analysis.

Also increase consistency with how overflow && zero is tested, and
 avoid some mixed declarations and code that GCC wasn't detecting.

src/scalar_4x64_impl.h

@@ -738,7 +738,7 @@ static void secp256k1_scalar_mul_512(uint64_t l[8], const secp256k1_scalar *a, c
     extract(l[5]);
     muladd_fast(a->d[3], b->d[3]);
     extract_fast(l[6]);
-    VERIFY_CHECK(c1 <= 0);
+    VERIFY_CHECK(c1 == 0);
     l[7] = c0;
 #endif
 }