Don't set iPicWidth or iPicHeight to 0 in random tests

This should fix occasional failures in the EncoderInterfaceTests,
such as in https://travis-ci.org/cisco/openh264/jobs/30815104.
Merge pull request #1222 from mstorsjo/fix-test-valid-size

Don't set iPicWidth or iPicHeight to 0 in random tests
Merge remote-tracking branch 'upstream/master' into travis.osx