Extend the API documentation for jerry_create_string_* functions (#3234)

- The given string pointer cannot be NULL.
 - Fixed API prototype for jerry_create_string_sz_from_utf8
 - Assert is added for non-NULL pointer in lit_zt_utf8_string_size

JerryScript-DCO-1.0-Signed-off-by: Robert Fancsik frobert@inf.u-szeged.hu

jerry-core/lit/lit-strings.c

@@ -247,13 +247,15 @@ convert_code_point_to_high_surrogate (lit_code_point_t code_point) /**< code poi
  * Calculate size of a zero-terminated utf-8 string
  *
  * NOTE:
- *   string should not contain zero characters in the middel
+ *   - string cannot be NULL
+ *   - string should not contain zero characters in the middle
  *
  * @return size of a string
  */
 lit_utf8_size_t
 lit_zt_utf8_string_size (const lit_utf8_byte_t *utf8_str_p) /**< zero-terminated utf-8 string */
 {
+  JERRY_ASSERT (utf8_str_p != NULL);
   return (lit_utf8_size_t) strlen ((const char *) utf8_str_p);
 } /* lit_zt_utf8_string_size */