fix various memory leaks

[yukiteruamano]

Hi, this changes fix a few memory leaks detected by clang-static-analyzer. The fixes are in backend/egl.c, backend/glx.c, src/config.c and transition/script.c

There are other issues detected, but touching them will break picom, so we'll have to keep checking these new fixes. At the moment, these fixes work perfectly on my machine (FreeBSD) for both amd64 and i386.

As I fix the rest of the bugs detected with clang-static-analyzer, I'll upload the rest of the commits.