rtos: add debug prints for qSymbol lookup

rtos: add debug prints for qSymbol lookup

- Added two debug prints that allow to inspect the symbol
  lookup process (carried out via the qSymbol packets).

- Added a check for an edge situation when the debugger
  would send us different symbol than what we asked for
  (avoids NULL dereference).

Change-Id: I84ae1b2b66473eab76a7966c678d2777b6ef48d6
Signed-off-by: Jan Matyas <matyas@codasip.com>
Reviewed-on: http://openocd.zylin.com/6287
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>