Make dc_msg_get_summarytext_by_raw safe (#316)

* Make dc_msg_get_summarytext_by_raw safe

* use dc_truncate method in all places

* Fix tests and add docs to dc_truncate()

* Make text argument an AsRef<&str> and rename type_0 to viewtype

* Fix too early return in dc_msg_get_summarytext_by_raw. Fixes https://github.com/deltachat/deltachat-core-rust/issues/313

src/dc_lot.rs

@@ -166,15 +166,14 @@ pub unsafe fn dc_lot_fill(
         }
     }
 
-    let message_text = (*msg).text.as_ref().unwrap();
-
     (*lot).text2 = dc_msg_get_summarytext_by_raw(
         (*msg).type_0,
-        message_text.strdup(),
+        (*msg).text.as_ref(),
         &mut (*msg).param,
         160,
         context,
-    );
+    )
+    .strdup();
 
     (*lot).timestamp = dc_msg_get_timestamp(msg);
     (*lot).state = (*msg).state;