Fix memory leak in vx_data2d_grib2 library.

[JohnHalleyGotway]

During development for the tcrmw tool, David found a memory leak in the vx_data2d_grib2 library. The read_grib2_record() function allocates an array of unsigned char's for storing the contents of GRIB2 records. However, it fails to deallocate.

The pointer for this memory is passed as an argument to this function, but that isn't necessary. Update the code to declare the pointer locally, allocated, and deallocate before returning.

[JohnHalleyGotway]

Fixed in the master_v8.1 and develop branches.