When the `jerry_substring_to_utf8_char_buffer` was called
with direct strings the assert incorrectly assumed that the
string is a ref counted string and tried to access the
refcount value resulting in a failed assert.
Added direct string check for the underlying implementation
and created a test case for such simple string.
JerryScript-DCO-1.0-Signed-off-by: Peter Gal [email protected]