Fixed several problems found by PVS-Studio.

This fixes all problems from the article "Checking OpenCV with PVS-Studio"
<http://www.viva64.com/en/b/0191/> that are not already fixed and are
not in 3rdparty or the legacy module.

The problems fixed are two instances of useless code and one instance
of unspecified behavior (right-shifting a negative number).
run.py: added --android_env parameter
run.py: propagate OPENCV* env variables only with --android_propagate_opencv_env flag
Merge pull request #2793 from alalek:run_android_env
2 fixed unassigned reshapes in em (#3674)
Merge pull request #2796 from berak:b_3674_24
Merge pull request #2771 from SpecLad:pvs-checks-opencv
Merge remote-tracking branch 'origin/2.4' into merge-2.4
Merge pull request #2819 from SpecLad:merge-2.4