zdb: Fix handling of nul termination in symlink targets

The SA attribute containing the symlink target does not include a nul
terminator, so when printing the target zdb would sometimes include
garbage at the end of the string.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #13482

cmd/zdb/zdb.c

@@ -3194,13 +3194,18 @@ dump_znode_symlink(sa_handle_t *hdl)
 {
 	int sa_symlink_size = 0;
 	char linktarget[MAXPATHLEN];
-	linktarget[0] = '\0';
 	int error;
 
 	error = sa_size(hdl, sa_attr_table[ZPL_SYMLINK], &sa_symlink_size);
 	if (error || sa_symlink_size == 0) {
 		return;
 	}
+	if (sa_symlink_size >= sizeof (linktarget)) {
+		(void) printf("symlink size %d is too large\n",
+		    sa_symlink_size);
+		return;
+	}
+	linktarget[sa_symlink_size] = '\0';
 	if (sa_lookup(hdl, sa_attr_table[ZPL_SYMLINK],
 	    &linktarget, sa_symlink_size) == 0)
 		(void) printf("\ttarget	%s\n", linktarget);