#335 [bugfix] Make TypeReturnNull.bugAccumulator attribute protected

[uhzzre]

Currently bugAccumulator attribute n TypeReturnNull class has no modifier which makes the field only accessible by files in the same package i.e. OptionalReturnNull and BooleanReturnNull.
By changing it to protected any subclass may use it e.g. one can write an external plugin that derives from the class.

[Tagir Valeev]

Fixed:
https://code.google.com/p/findbugs/source/detail?r=78751f38ff0177635a62a6e8de5d02387fa85aa6