YANG parser BUGFIX improve checking of input data when parsing

Despite the testcase is connected with special UTF8 character, it is
actually detected as unexpected end-of-input since the character starts
with a NULL-byte. In such a case libyang is not able to detect that
after the NULL-byte are still some data to read, it just interpret it as
end of input string.

Other, not-supported (by RFC) UTF8 characters should be detected
correctly with different error message.

Fixes #780
diff --git a/tests/src/test_xml.c b/tests/src/test_xml.c
index 671a1cf..22fc3e6 100644
--- a/tests/src/test_xml.c
+++ b/tests/src/test_xml.c
@@ -316,13 +316,13 @@
     str = "";
     assert_int_equal(LY_EVALID, lyxml_get_string(&ctx, &str, &buf, &buf_len, &out, &len, &dynamic));
     assert_null(buf);
-    logbuf_assert("Unexpected end-of-file. Line number 2.");
+    logbuf_assert("Unexpected end-of-input. Line number 2.");
 
     ctx.status = LYXML_ELEM_CONTENT;
     str = p = "xxx";
     assert_int_equal(LY_EVALID, lyxml_get_string(&ctx, &str, &buf, &buf_len, &out, &len, &dynamic));
     assert_null(buf);
-    logbuf_assert("Unexpected end-of-file. Line number 2.");
+    logbuf_assert("Unexpected end-of-input. Line number 2.");
     assert_ptr_equal(p, str); /* input data not eaten */
 
     /* valid strings */