Fix schemagen integer parser rejecting zero with negative exponents

Move the zero-detection check in the generated parseJsonInt64 before the
finalExp < 0 guard. Previously, valid JSON numbers whose mathematical
value is 0 but written with a large negative exponent (e.g. 0e-2,
0.0e-2, -0e-2, 0e-20) were rejected because the negative-finalExp early
return ran before the all-zero-digits branch could set out = 0. Non-zero
values with negative exponents are still correctly rejected.

Closes #56

contrib/schemagen/weaseljson_schemagen.py

@@ -1058,10 +1058,10 @@ private:
       ++trim;
     }}
     int64_t finalExp = exp - fracDigits + trim;
-    if (finalExp < 0) return false;
     size_t leadingZeros = 0;
     while (leadingZeros < digits.size() && digits[leadingZeros] == '0') ++leadingZeros;
     if (leadingZeros == digits.size()) {{ out = 0; return true; }}
+    if (finalExp < 0) return false;
     if (leadingZeros > 0) digits.erase(0, leadingZeros);
 
     constexpr uint64_t kMaxNeg = 9223372036854775808ULL;