Unmasked data when using DayNightCompositor

[pnuu]

When using DayNightCompositor and produce an image in a global EPSG:4326 grid there's a unmasked (black) area around the valida data area. If the sub-images used within the DayNightCompositor are loaded, resampled and saved, they have the areas masked correctly.

To Reproduce

areas.def:

REGION: EPSG4326 {
   NAME:    Global equal latitude/longitude grid for global sphere
   PCS_ID:  EPSG:4326
   PCS_DEF: init=EPSG:4326
   XSIZE: 8192
   YSIZE: 4096
   AREA_EXTENT:  (-180.0, -90.0, 180.0, 90.0)
};

$PPP_CONFIG_DIR/composites/seviri.yaml:

  natural_with_night_fog:
    compositor: !!python/name:satpy.composites.DayNightCompositor
    standard_name: natural_with_night_fog
    prerequisites:
      - natural
      - night_fog

import glob
from satpy import Scene

fnames = glob.glob('/data/*201805220700*')
glbl = Scene(reader='hrit_msg', filenames=fnames)
glbl.load(['natural_with_night_fog'])
lcl = glbl.resample('EPSG4326', resampler='nearest')
lcl.save_dataset('natural_with_night_fog', '/tmp/natural_with_night_fog.png')

Expected behavior
Image with properly masked out-of-area, like in the image below (composite from 0.6 channel, doesn't matter which one used).

Actual results
Unmasked area around the valid data shouldn't be there.

Environment Info:

• OS: Linux, RHEL6, Conda, Python 3.6
• SatPy Version: 0.9.1a0.dev0
• PyResample Version: 1.10.1