Evas textblock: Skip layout logic for an text item which doesn't have font.

Evas textblock: Skip layout logic for an text item which doesn't have font.

Summary:
Evas textblock could cause infinite loop if there is no fonts to use.
If there is no fonts, text_props.text_len is never set.
When text_props.text_len is 0, the for loop in _layout_par runs forever.
It is ridiculous to use Textblock without fonts. But, it shouldn't runs
infinite loop in any situation.

@fix

Test Plan:
1. Remove all of fonts in your EFL or Tizen device.
   (Or you can test it modifying some codes in Textblock by skipping load fonts.)
2. Run elementary_test -to entry3 or see any multiline textblocks.

Reviewers: tasn, herdsman, woohyun

Subscribers: cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D3402