Clang10 UBSAN has a new warning about applying offset to nullptr,
this is undefined behaviour.

This patch fixes most such issues in the default test suite with --big-test,
by testing for nullptr before adding any offset.

The check for nullptr offset also uncovered missing initialization
of IO_CACHE objects used in HashJoinChunk.

We add a new macro SUPPRESS_UBSAN_CLANG10 for issues that are not fixed
by this patch (separate bugs have been filed).

Change-Id: I94bdb601f2a1d1c9054501c15af5f3245aef3ab5
(cherry picked from commit 9017b09a5efbbdcfdb5474b2c6b0cdf0bae3fecb)