Add NULL check routine for frame on FrameView::scrollTo (N_SE-36624)

Add NULL check routine for frame on FrameView::scrollTo (N_SE-36624)

 [Title] Add NULL check routine for frame on FrameView::scrollTo
 [Issue] N_SE-36624 (same with http://slp-info.sec.samsung.net/gerrit/#/c/179408/)
 [Problem] Segmentation Fault on FrameView::scrollPositionChanged() because the frame() is NULL.
 [Cause]
  FrameView::scrollTo was called from resetScrollbars during FrameView destruction.
  In this case, because the frame() can be NULL, the below codes that access the frame() shouldn't be executed.
 [Solution] Add NULL check routine.

Change-Id: I7eb507659901fd28c3e18969f3798cf84a1062b6