color: Fix an assertion failure with embedded NUL bytes

Leading NUL bytes (e.g. `*\0.gz=...`) could cause us to insert a
non-prefix-free set of strings into the case-insensitive extension trie,
which would lead to crashes like

   bfs: trie_split@src/trie.c:538: Assertion failed: `key_nibble != rep_nibble`

and OOB accesses to trie leaf keys.  Fix it by ignoring those
extensions, since filenames cannot contain NUL bytes.

Fixes: 08030ae ("color: Delay the case_sensitive decision")

src/color.c

@@ -237,6 +237,16 @@ static int insert_ext(struct trie *trie, struct ext_color *ext) {
 /** Set the color for an extension. */
 static int set_ext(struct colors *colors, dchar *key, dchar *value) {
 	size_t len = dstrlen(key);
+
+	// Embedded NUL bytes in extensions can lead to a non-prefix-free
+	// set of strings, e.g. {".gz", "\0.gz"} would be transformed to
+	// {"zg.\0", "zg.\0\0"} (showing the implicit terminating NUL).
+	// Our trie implementation only supports prefix-free key sets, but
+	// luckily '\0' cannot appear in filenames so we can ignore them.
+	if (memchr(key, '\0', len)) {
+		return 0;
+	}
+
 	struct ext_color *ext = varena_alloc(&colors->ext_arena, len + 1);
 	if (!ext) {
 		return -1;

tests/bfs/color_ext_case_nul.out

@@ -0,0 +1,27 @@
+$'rainbow/\e[1m'
+$'rainbow/\e[1m/'$'\e[0m'
+rainbow
+rainbow/lower.gz
+rainbow/lower.tar.gz
+rainbow/lu.tar.GZ
+rainbow/ul.TAR.gz
+rainbow/upper.GZ
+rainbow/upper.TAR.GZ
+rainbow/exec.sh
+rainbow/socket
+rainbow/broken
+rainbow/chardev_link
+rainbow/link.txt
+rainbow/sticky_ow
+rainbow/sgid
+rainbow/pipe
+rainbow/ow
+rainbow/sugid
+rainbow/suid
+rainbow/sticky
+rainbow/file.dat
+rainbow/file.txt
+rainbow/lower.tar
+rainbow/mh1
+rainbow/mh2
+rainbow/upper.TAR

tests/bfs/color_ext_case_nul.sh

@@ -0,0 +1,5 @@
+# Regression test: embedded NUL bytes in an extension caused an assertion
+# failure in the trie implementation
+
+export LS_COLORS='*.gz=01;31:*\0.GZ=01;32:'
+bfs_diff rainbow -color