libsepol: do not crash on user gaps

Handle gaps in the user table while printing a policy configuration.

    ==24424==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x0000004bdc55 bp 0x7ffc8790b810 sp 0x7ffc8790afb0 T0)
    ==24424==The signal is caused by a READ memory access.
    ==24424==Hint: address points to the zero page.
        #0 0x4bdc55 in __interceptor_strcmp (./out/binpolicy-fuzzer+0x4bdc55)
        #1 0x5ebdf6 in strs_cmp ./libsepol/src/kernel_to_common.c:253:9
        #2 0x505669 in __interceptor_qsort (./out/binpolicy-fuzzer+0x505669)
        #3 0x5ebd84 in strs_sort ./libsepol/src/kernel_to_common.c:261:2
        #4 0x564550 in write_user_decl_rules_to_conf ./libsepol/src/kernel_to_conf.c:2333:2
        #5 0x55b137 in sepol_kernel_policydb_to_conf ./libsepol/src/kernel_to_conf.c:3190:7
        #6 0x55a34f in LLVMFuzzerTestOneInput ./libsepol/fuzz/binpolicy-fuzzer.c:38:9
        #7 0x45aed3 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) fuzzer.o
        #8 0x446a12 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) fuzzer.o
        #9 0x44c93b in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) fuzzer.o
        #10 0x475dd2 in main (./out/binpolicy-fuzzer+0x475dd2)
        #11 0x7f530128d7ec in __libc_start_main csu/../csu/libc-start.c:332:16
        #12 0x423689 in _start (./out/binpolicy-fuzzer+0x423689)

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libsepol/src/kernel_to_cil.c

@@ -2397,6 +2397,7 @@ static int write_user_decl_rules_to_cil(FILE *out, struct policydb *pdb)
 	}
 
 	for (i=0; i < pdb->p_users.nprim; i++) {
+		if (!pdb->p_user_val_to_name[i]) continue;
 		rc = strs_add(strs, pdb->p_user_val_to_name[i]);
 		if (rc != 0) {
 			goto exit;

libsepol/src/kernel_to_conf.c

@@ -2327,6 +2327,7 @@ static int write_user_decl_rules_to_conf(FILE *out, struct policydb *pdb)
 	}
 
 	for (i=0; i < pdb->p_users.nprim; i++) {
+		if (!pdb->p_user_val_to_name[i]) continue;
 		rc = strs_add(strs, pdb->p_user_val_to_name[i]);
 		if (rc != 0) {
 			goto exit;