Return 0 if binary exponent is too small

include/rapidjson/internal/diyfp.h

@@ -142,7 +142,10 @@ struct DiyFp {
             uint64_t u64;
         }u;
         RAPIDJSON_ASSERT(f <= kDpHiddenBit + kDpSignificandMask);
-        RAPIDJSON_ASSERT(e >= kDpDenormalExponent);
+        if (e < kDpDenormalExponent) {
+            // Underflow.
+            return 0.0;
+        }
         RAPIDJSON_ASSERT(e < kDpMaxExponent);
         const uint64_t be = (e == kDpDenormalExponent && (f & kDpHiddenBit) == 0) ? 0 :
             static_cast<uint64_t>(e + kDpExponentBias);

test/unittest/readertest.cpp

@@ -392,6 +392,15 @@ static void TestParseDouble() {
         "83723677529752585477247372368372368547354737253685475529752",
         6223372036854775808.0);
 
+
+    TEST_DOUBLE(fullPrecision, "1e-325", 0.0);
+    TEST_DOUBLE(fullPrecision, "1e-324", 0.0);
+    TEST_DOUBLE(fullPrecision, "2e-324", 0.0);
+    TEST_DOUBLE(fullPrecision, "2.4703282292062327e-324", 0.0);
+    TEST_DOUBLE(fullPrecision, "2.4703282292062328e-324", 2.4703282292062328e-324);
+    TEST_DOUBLE(fullPrecision, "2.48e-324", 2.48e-324);
+    TEST_DOUBLE(fullPrecision, "2.5e-324", 2.5e-324);
+
 #undef TEST_DOUBLE
 }