fix crash on some mailto: links

if the value part is missing,
it otherwise crashes when calling MailtoUtil.getText()
and the exception is not handled there (it is not in at least one case).

therefore, just catch the exception in this cornercase (same for bad encoding)
and return an empty string.

src/main/java/org/thoughtcrime/securesms/util/MailtoUtil.java

@@ -54,7 +54,7 @@ public class MailtoUtil {
                 String[] queryEntryArray = queryEntry.split(KEY_VALUE_SEPARATOR);
                 try {
                     mailtoQueryMap.put(queryEntryArray[0], URLDecoder.decode(queryEntryArray[1], "UTF-8"));
-                } catch (UnsupportedEncodingException e) {
+                } catch (Exception e) {
                     e.printStackTrace();
                 }
             }