Handle Bigint.toCString when remainder equals 0.

runtime/vm/bigint_operations.cc

Index: runtime/vm/bigint_operations.cc
diff --git a/runtime/vm/bigint_operations.cc b/runtime/vm/bigint_operations.cc
index 71b593bdb4b436199afa7116bd5e84bab1b39802..5c633d01061dbf6ed2f193382ad3eb4c32c9cd20 100644
--- a/runtime/vm/bigint_operations.cc
+++ b/runtime/vm/bigint_operations.cc
@@ -390,8 +390,10 @@ const char* BigintOperations::ToDecimalCString(
   Bigint& remainder = Bigint::Handle();
   while (!rest.IsZero()) {
     DivideRemainder(rest, divisor, &quotient, &remainder);
-    ASSERT(remainder.Length() == 1);
-    intptr_t part = static_cast<intptr_t>(remainder.GetChunkAt(0));
+    ASSERT(remainder.Length() <= 1);
+    intptr_t part = (remainder.Length() == 1)
+        ? static_cast<intptr_t>(remainder.GetChunkAt(0))
+        : 0;
     for (int i = 0; i < kPowerOfTen; i++) {
       result[result_pos++] = '0' + (part % 10);
       part /= 10;