fix: util.global = global when this is undefined

[ZhiruiLi]

this may be undefined in strict mode, because source code may be wrapped by some lambda function before being executed. In this case, error will occur in the following code:

util.Long = /* istanbul ignore next */ util.global.dcodeIO && /* istanbul ignore next */ util.global.dcodeIO.Long
         || /* istanbul ignore next */ util.global.Long
         || util.inquire("long");

Instead keeps the util.global as undefined, we assign user defined global to util.global. This can be very useful when using protobuf.js in some specifical env (for example: game engine).