PNM decoder: prevent unsupported depth=2 PAM case.
PNM decoder: prevent unsupported depth=2 PAM case.

Change-Id: I8476818908d71498dd80b07dc255aa008ffd16f5

webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp