yanglint BUGFIX missing NULL check

Fixes #1825
diff --git a/tools/lint/common.c b/tools/lint/common.c
index b41e4fe..c2e2208 100644
--- a/tools/lint/common.c
+++ b/tools/lint/common.c
@@ -520,7 +520,7 @@
             lyd_print_all(out, merged_tree, format, options_print);
         }
 
-        for (uint32_t u = 0; u < xpaths->count; ++u) {
+        for (uint32_t u = 0; xpaths && (u < xpaths->count); ++u) {
             if (evaluate_xpath(merged_tree, (const char *)xpaths->objs[u])) {
                 goto cleanup;
             }