Meta: Enable nonnull-compare warnings

Previously, this was disabled because GCC flagged seemingly correct and
well-defined code. This was however not the case because GCC implicitly
marked some pointers non-null, even if we wanted to handle them
ourselves, and deleted null checks on them. By re-introducing this
warning, we will know if the compiler tries to discard our code again.

CMakeLists.txt

@@ -253,7 +253,6 @@ add_compile_options(-Wimplicit-fallthrough)
 add_compile_options(-Wlogical-op)
 add_compile_options(-Wmisleading-indentation)
 add_compile_options(-Wmissing-declarations)
-add_compile_options(-Wno-nonnull-compare)
 add_compile_options(-Wnon-virtual-dtor)
 add_compile_options(-Wno-unknown-warning-option)
 add_compile_options(-Wundef)