Do not segfault on MacOSX when the java executable could not be found

Reported by Ben W Booth in

	https://github.com/imagej/imagej-launcher/issues/19

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Ignore double colons in the PATH

When looking for an executable in the PATH, let's not stumble when
encountering an empty element, i.e. a double colon.

Reported by Ben W Booth in

	https://github.com/imagej/imagej-launcher/issues/19

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'macosx-path'

This topic branch fixes the problems implied by the bug report at

	https://github.com/imagej/imagej-launcher/issues/19

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>