win_alloc_firstwin can still FAIL (squash into window.c)
Add proper function attributes to the new functions in memory.h
No OOM error in call_shell() and read_string()
No OOM error conditions in quickfix.c and regex_nfa.c

quickfix.c

 - ll_new_list
 - ll_get_or_alloc_list

regex_nfa.c

 - realloc_post_list -> EMIT -> nfa_emit_equi_class
 - nfa_regcomp_start
Add FUNC_ATTR_NONNULL_RET to some functions