Null terminate exponent buffer.

Fixes issue 26.

src/double-conversion.cc

@@ -98,7 +98,8 @@ void DoubleToStringConverter::CreateExponentialRepresentation(
   }
   ASSERT(exponent < 1e4);
   const int kMaxExponentLength = 5;
-  char buffer[kMaxExponentLength];
+  char buffer[kMaxExponentLength + 1];
+  buffer[kMaxExponentLength] = '\0';
   int first_char_pos = kMaxExponentLength;
   while (exponent > 0) {
     buffer[--first_char_pos] = '0' + (exponent % 10);