Fix failure to consider failure cases in GetComboCommandId().

Failure to initially palloc the comboCids array, or to realloc it bigger
when needed, left combocid's data structures in an inconsistent state that
would cause trouble if the top transaction continues to execute.  Noted
while examining a user complaint about the amount of memory used for this.
(There's not much we can do about that, but it does point up that repalloc
failure has a non-negligible chance of occurring here.)

In HEAD/9.5, also avoid possible invocation of memcpy() with a null pointer
in SerializeComboCIDState; cf commit 13bba022.