YANG parser BUGFIX keyword as a prefix name in extension instance

keyword followed by colon is not a keyword, but a prefix of a possible
extension instance.
diff --git a/tests/src/test_parser_yang.c b/tests/src/test_parser_yang.c
index 56404e0..78cf9ef 100644
--- a/tests/src/test_parser_yang.c
+++ b/tests/src/test_parser_yang.c
@@ -801,6 +801,12 @@
     assert_int_equal(2, mod->version);
     mod = mod_renew(&ctx, mod, 0);
 
+    /* extensions */
+    TEST_GENERIC("prefix:test;}", mod->exts,
+                 assert_string_equal("prefix:test", mod->exts[0].name);
+                 assert_int_equal(LYEXT_SUBSTMT_SELF, mod->exts[0].insubstmt));
+    mod = mod_renew(&ctx, mod, 0);
+
     /* submodule */
     mod->submodule = 1;