libkmod: fix address argument to mmap calls

The first argument to mmap should be "NULL" instead of "0".
diff --git a/libkmod/libkmod-index.c b/libkmod/libkmod-index.c
index 516240e..d386f00 100644
--- a/libkmod/libkmod-index.c
+++ b/libkmod/libkmod-index.c
@@ -801,9 +801,9 @@
 	if ((size_t) st.st_size < sizeof(hdr))
 		goto fail_nommap;
 
-	if ((idx->mm = mmap(0, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0))
+	if ((idx->mm = mmap(NULL, st.st_size, PROT_READ, MAP_PRIVATE, fd, 0))
 							== MAP_FAILED) {
-		ERR(ctx, "mmap(0, %"PRIu64", PROT_READ, %d, MAP_PRIVATE, 0): %m\n",
+		ERR(ctx, "mmap(NULL, %"PRIu64", PROT_READ, %d, MAP_PRIVATE, 0): %m\n",
 							st.st_size, fd);
 		goto fail_nommap;
 	}