Fix warnings with gcc 7.3 and clang 5.0

Description

Fix warnings from pc-chritophe:

https://dash.orfeo-toolbox.org/index.php?project=OTB

Steps to reproduce

Describe as precisely as possible how to reproduce the bug. Try to isolate a minimal number of steps. Also describe reproducibility (always, random ...).

Configuration information

OS, OTB version or tag, information related to build (binaries, superbuild, system libs ...)