Fix missing null check in Clipboard::ReadImage.

Fix missing null check in Clipboard::ReadImage.

Even though CF_DIB is always supposed to be synthesized from CF_BITMAP, change
the types to match in Clipboard::ReadAvailableTypes. We also add a NULL check
in Clipboard::ReadImage in case the contents of the clipboard change between the
time the clipboard types are accessed and the the clipboard content is accessed.

BUG=94332
TEST=none

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=98361

[dcheng, 2011-08-25 23:48:00]

I lost the null check when I was refactoring things to fix the other bug.

[tony, 2011-08-25 23:59:39]

LGTM