screen_ui: Initialize text buffer

Zero initialize the text buffer to make recovery not
render garbage when showing the menu or messages.

Change-Id: I0dd0d357757f6b0fd52ad3b3617d42bb1b835245

screen_ui.cpp

@@ -86,6 +86,8 @@ ScreenRecoveryUI::ScreenRecoveryUI() :
     for (int i = 0; i < 5; i++)
         backgroundIcon[i] = NULL;
 
+    memset(text, 0, sizeof(text));
+
     pthread_mutex_init(&updateMutex, NULL);
     self = this;
 }