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/num_gmp_impl.h

@@ -232,12 +232,12 @@ static void secp256k1_num_mul(secp256k1_num *r, const secp256k1_num *a, const se
 }
 
 static void secp256k1_num_shift(secp256k1_num *r, int bits) {
-    int i;
     if (bits % GMP_NUMB_BITS) {
         /* Shift within limbs. */
         mpn_rshift(r->data, r->data, r->limbs, bits % GMP_NUMB_BITS);
     }
     if (bits >= GMP_NUMB_BITS) {
+        int i;
         /* Shift full limbs. */
         for (i = 0; i < r->limbs; i++) {
             int index = i + (bits / GMP_NUMB_BITS);