r.horizon: address coverity scan 'Null pointer dereferences'

[petrasovaa]

This should address this (I think)

*** CID 1543994:  Null pointer dereferences  (REVERSE_INULL)
/raster/r.horizon/main.c: 1274 in calculate_raster_mode()
1268             Rast_append_format_history(
1269                 &history,
1270                 "Horizon view from azimuth angle %.2f degrees CCW from East",
1271                 angle * rad2deg);
1272     
1273             Rast_write_history(shad_filename, &history);
>>>     CID 1543994:  Null pointer dereferences  (REVERSE_INULL)
>>>     Null-checking "shad_filename" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
1274             if (shad_filename)
1275                 G_free(shad_filename);
1276         }

[nilason]

I believe this should do it, shad_filename was never freed after its allocation. Even if it happened to be still NULL then free() wouldn't do anything.