option java_multiple_files breaks ProtoTypeMap

[efenglu]

Create a proto file with option java_multiple_files=true

option java_multiple_files = true;

message Outer {  // Level 0
    enum FooEnum {
        FOO = 0;
        BAR = 1;
        CHEESE = 2;
    }
    message MiddleAA {  // Level 1

        message Inner {   // Level 2
            int64 ival = 1;
            bool  booly = 2;
            Outer.FooEnum enum = 3;
        }
    }
    message MiddleBB {  // Level 1
        message Inner {   // Level 2
            int32 ival = 1;
            bool  booly = 2;
            Outer.FooEnum enum = 3;
        }
    }
}

The resulting map has incorrect paths and 'null' string literals in path.