Nullptr check for getActiveConfig() in doDump()

HWComposer::getActiveConfig() returns nullptr when a bad
configuration is received from HWC. In this case a dumpsys
will cause a nullptr dereference and it'll crash SurfaceFlinger.
This CL adds a nullptr check to prevent this.

Bug: 165419673
Test: adb shell dumpsys SurfaceFlinger
Change-Id: I7da8d034a5a429412a73121d795e3adc4323db13