Revert "Remove unnecessary null check.  NFC."
Revert "Remove unnecessary null check.  NFC."

This reverts commit r243167.

Duncan pointed out that dyn_cast can return null in these cases, so this
was an unsafe commit to make.  Sorry for the noise.

Worryingly there were no tests which fail...

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@243302 91177308-0d34-0410-b5e6-96231b3b80d8

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