Debugger: Fix Step into for unreachable frame source.

Hitting a frame without source code information when stepping inside
functions results in a step out. Same is now applied to frames with
unreachable source code.

Task-number: QTCREATORBUG-9677
Change-Id: If7f7b417d98e8ab058fc98cef3041cb616579619
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>