out-of-bounds access on Enter in new text field

https://bugzilla.redhat.com/show_bug.cgi?id=1575842

Reproduce using:
1. Select text tool (F8)
2. Click on empty canvas
3. Hit Enter

Backport from 1.0alpha master
Cherry picked from 37a5a0e7

src/libnrtype/Layout-TNG-OutIter.cpp

@@ -182,6 +182,8 @@ Layout::iterator Layout::sourceToIterator(void *source_cookie /*, Glib::ustring:
     if (_input_stream[source_index]->Type() != TEXT_SOURCE)
         return iterator(this, char_index);
 
+    if (char_index >= _characters.size())
+        return end();
     return iterator(this, char_index);
     /* This code was never used, the text_iterator argument was "NULL" in all calling code
     InputStreamTextSource const *text_source = static_cast<InputStreamTextSource const *>(_input_stream[source_index]);