common CHANGE new ly_strncmp()

compare NULL-terminated reference string with a number of bytes from
other string.
diff --git a/src/parser_yang.c b/src/parser_yang.c
index e6830d3..314cbd7 100644
--- a/src/parser_yang.c
+++ b/src/parser_yang.c
@@ -2171,7 +2171,7 @@
         return LY_EVALID;
     }
 
-    if (strncmp(word, "unbounded", word_len)) {
+    if (ly_strncmp("unbounded", word, word_len)) {
         errno = 0;
         num = strtoul(word, &ptr, 10);
         /* we have not parsed the whole argument */