Update picojson_serializer.h to avoid GCC uninitialized warnings.

This change was required to fix GCC uninitialized variable warnings. I have a class with uint32_t members which generated "Maybe unitialized" warnings with GCC-4.7.3. The uint32_t class member is initialized to 0 using an initializer list yet the warning persisted. 

Eventually I came across this StackOverflow post that basically hinted towards optimization -O2 as being the root cause. It's the answer by juanchopanza, not the accepted answer. http://stackoverflow.com/questions/20909610/default-initialization-of-aggregates

I quote.

"In order to guarantee zero-initialization of POD aggregate data members, you need to value-initialize your instance. For example,

st s = st(); // C++03 and C++11
st s = {};   // C++03 and C++11
st s{};      // C++11
"

After adding {} after t, my warnings disappeared and all seems well. No other impact on other classes being serialized. Figured I'd submit this patch to assist others who code "warning-free" :)