As reported by cppcheck:
src/list.c:45:8: style: Redundant initialization for 'data'. The
initialized value is overwritten before it is read.
[redundantInitialization]
data = NULL;
^
src/list.c:43:15: note: data is initialized
void * data = list_pop_front(list);
^
src/list.c:45:8: note: data is overwritten
data = NULL;
^