Clang: enable -Werror and fix code

(cherry picked from commit 4f50848d99e76900f29ed8e1da55edab2e632b7b)

Conflicts:
	src/CMakeLists.txt
	src/views/slideshow.c
No need for strlen here

(cherry picked from commit b6e851317bf4eea572ed6491422a67d9d2a1e873)
Do not compare array against 0
Coverity CID 1111070, CID 1034348, CID 1034347, CID 1034345

(cherry picked from commit cd9d547f25e7b448e7a5f7897918f0c48b4b2e0a)