libsepol: reject invalid filetrans source type

Avoid integer underflow on invalid filetrans source types.

    policydb.c:2658:47: runtime error: unsigned integer overflow: 0 - 1 cannot be represented in type 'unsigned int'
        #0 0x4cf4cb in policydb_filetrans_insert ./libsepol/src/policydb.c:2658:47
        #1 0x4d221a in filename_trans_read_one_compat ./libsepol/src/policydb.c:2691:7
        #2 0x4d221a in filename_trans_read ./libsepol/src/policydb.c:2842:9
        #3 0x4d1370 in policydb_read ./libsepol/src/policydb.c:4447:7
        #4 0x4b1ee3 in LLVMFuzzerTestOneInput ./libsepol/fuzz/binpolicy-fuzzer.c:35:6
        #5 0x43f2f3 in fuzzer::Fuzzer::ExecuteCallback(unsigned char const*, unsigned long) fuzzer.o
        #6 0x42ae32 in fuzzer::RunOneTest(fuzzer::Fuzzer*, char const*, unsigned long) fuzzer.o
        #7 0x430d5b in fuzzer::FuzzerDriver(int*, char***, int (*)(unsigned char const*, unsigned long)) fuzzer.o
        #8 0x45a1f2 in main (./out/binpolicy-fuzzer+0x45a1f2)
        #9 0x7f8b8923a7ec in __libc_start_main csu/../csu/libc-start.c:332:16
        #10 0x407aa9 in _start (./out/binpolicy-fuzzer+0x407aa9)

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

libsepol/src/policydb.c

@@ -2683,7 +2683,10 @@ static int filename_trans_read_one_compat(policydb_t *p, struct policy_file *fp)
 	if (rc < 0)
 		goto err;
 
-	stype  = le32_to_cpu(buf[0]);
+	stype = le32_to_cpu(buf[0]);
+	if (stype == 0)
+		goto err;
+
 	ttype  = le32_to_cpu(buf[1]);
 	tclass = le32_to_cpu(buf[2]);
 	otype  = le32_to_cpu(buf[3]);