GenericValue::FindMember: avoid buffer overflow

In GenericValue::FindMember, the 'name' argument may be dereferenced
beyond its length, depending on the lengths of the currently existing
member names in the JSON object.

Instead of unconditionally dereferencing the given string, check for
its length first and skip comparisons for different lengths.

Fixes http://code.google.com/p/rapidjson/issues/detail?id=108.