Fixes parsing small floating point values underflow

https://code.google.com/p/rapidjson/issues/detail?id=75

The modification is slightly different from dlbattle123 to improve
speed.

include/rapidjson/reader.h

@@ -664,7 +664,15 @@ private:
 
 		// Finish parsing, call event according to the type of number.
 		if (useDouble) {
-			d *= internal::Pow10(exp + expFrac);
+			int expSum = exp + expFrac;
+			if (expSum < -308) {
+				// Prevent expSum < -308, making Pow10(expSum) = 0
+				d *= internal::Pow10(exp);
+				d *= internal::Pow10(expFrac);
+			}
+			else
+				d *= internal::Pow10(expSum);
+
 			handler.Double(minus ? -d : d);
 		}
 		else {

test/unittest/readertest.cpp

@@ -129,9 +129,10 @@ TEST(Reader, ParseNumberHandler) {
 	TEST_DOUBLE("1.234E+10", 1.234E+10);
 	TEST_DOUBLE("1.234E-10", 1.234E-10);
 	TEST_DOUBLE("1.79769e+308", 1.79769e+308);
-	//TEST_DOUBLE("2.22507e-308", 2.22507e-308);	// TODO: underflow
+	__asm int 3;
+	TEST_DOUBLE("2.22507e-308", 2.22507e-308);
 	TEST_DOUBLE("-1.79769e+308", -1.79769e+308);
-	//TEST_DOUBLE("-2.22507e-308", -2.22507e-308);	// TODO: underflow
+	TEST_DOUBLE("-2.22507e-308", -2.22507e-308);
 	TEST_DOUBLE("18446744073709551616", 18446744073709551616.0);	// 2^64 (max of uint64_t + 1, force to use double)
 	TEST_DOUBLE("-9223372036854775809", -9223372036854775809.0);	// -2^63 - 1(min of int64_t + 1, force to use double)