libsepol/cil: fix NULL pointer dereference with empty macro argument

OSS-Fuzz found a Null-dereference READ in the CIL compiler when trying
to compile the following policy:

    (macro m((name n))) (call m(()))

When calling the macro, the name (in variable "pc") is NULL, which
triggers a NULL pointer dereference when using it as a key in
__cil_insert_name(). The stack trace is:

    #0 0x7f4662655a85 in __strlen_avx2 (/usr/lib/libc.so.6+0x162a85)
    SELinuxProject#1 0x556d0b6d150c in __interceptor_strlen.part.0 (/selinux/libsepol/fuzz/fuzz-secilc+0x44850c)
    SELinuxProject#2 0x556d0ba74ed6 in symhash /selinux/libsepol/src/symtab.c:22:9
    SELinuxProject#3 0x556d0b9ef50d in hashtab_search /selinux/libsepol/src/hashtab.c:186:11
    SELinuxProject#4 0x556d0b928e1f in cil_symtab_get_datum /selinux/libsepol/src/../cil/src/cil_symtab.c:121:37
    SELinuxProject#5 0x556d0b8f28f4 in __cil_insert_name /selinux/libsepol/src/../cil/src/cil_resolve_ast.c:96:2
    SELinuxProject#6 0x556d0b908184 in cil_resolve_call1 /selinux/libsepol/src/../cil/src/cil_resolve_ast.c:2835:12
    SELinuxProject#7 0x556d0b91b404 in __cil_resolve_ast_node /selinux/libsepol/src/../cil/src/cil_resolve_ast.c
    SELinuxProject#8 0x556d0b91380f in __cil_resolve_ast_node_helper /selinux/libsepol/src/../cil/src/cil_resolve_ast.c:3773:7
    SELinuxProject#9 0x556d0b932230 in cil_tree_walk_core /selinux/libsepol/src/../cil/src/cil_tree.c:263:9
    SELinuxProject#10 0x556d0b932230 in cil_tree_walk /selinux/libsepol/src/../cil/src/cil_tree.c:307:7
    SELinuxProject#11 0x556d0b932326 in cil_tree_walk_core /selinux/libsepol/src/../cil/src/cil_tree.c:275:9
    SELinuxProject#12 0x556d0b932326 in cil_tree_walk /selinux/libsepol/src/../cil/src/cil_tree.c:307:7
    SELinuxProject#13 0x556d0b911189 in cil_resolve_ast /selinux/libsepol/src/../cil/src/cil_resolve_ast.c:3941:8
    SELinuxProject#14 0x556d0b798729 in cil_compile /selinux/libsepol/src/../cil/src/cil.c:550:7

Fixes: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=28544
Signed-off-by: Nicolas Iooss <nicolas.iooss@m4x.org>

libsepol/cil/src/cil_resolve_ast.c

@@ -2817,7 +2817,7 @@ int cil_resolve_call1(struct cil_tree_node *current, void *extra_args)
 		cil_list_for_each(item, new_call->macro->params) {
 			enum cil_flavor flavor = ((struct cil_param*)item->data)->flavor;
 
-			if (pc == NULL) {
+			if (pc == NULL || pc->data == NULL) {
 				cil_tree_log(current, CIL_ERR, "Missing arguments");
 				rc = SEPOL_ERR;
 				goto exit;