pgtable-test: Check correct error code for mmap

kvmtest/pgtable-test/dirty_logging.c

diff --git a/kvmtest/pgtable-test/dirty_logging.c b/kvmtest/pgtable-test/dirty_logging.c
index 712fe2d..5d9d1e2 100644
--- a/kvmtest/pgtable-test/dirty_logging.c
+++ b/kvmtest/pgtable-test/dirty_logging.c
@@ -48,7 +48,7 @@
 
 	mem_code = mmap(NULL, GUEST_MEM_SIZE, PROT_READ | PROT_WRITE,
 			MAP_SHARED | MAP_ANONYMOUS, -1, 0);
-	if (!mem_code)
+	if (mem_code == MAP_FAILED)
 		err(-ENOMEM, "allocating guest memory");
 	memcpy(mem_code, guest_code, GUEST_NR_INST * ARM64_INST_SIZE);