make it compile with glibc-2.26 with an explicit cast to bool

[bulwahn]

With glibc-2.26, openslam_gmapping failed to compile with:

| [...]/gridfastslam/gfsreader.cpp: In member function 'virtual void GMapping::GFSReader::RawOdometryRecord::read(std::istream&)':
| [...]/gridfastslam/gfsreader.cpp:79:3: error: no match for 'operator==' (operand types are 'std::istream {aka std::basic_istream<char>}' and 'int')
|    assert(is);
|    ^

Hence, this commit explicitly casts to bool, so that the assert annotation compiles properly again. The issue was discovered during the maintenance of meta-ros, the OpenEmbedded layer for ROS.