Add more assertions in palloc0() and palloc_extended()

Add more assertions in palloc0() and palloc_extended()

palloc() includes an assertion checking that an alloc() implementation
never returns NULL for all MemoryContextMethods.

This commit adds a similar assertion in palloc0().  In palloc_extend(),
a different assertion is added, checking that MCXT_ALLOC_NO_OOM is set
when an alloc() routine returns NULL.  These additions can be useful to
catch errors when implementing a new set of MemoryContextMethods
routines.

Author: Andreas Karlsson 
Discussion: https://postgr.es/m/507e8eba-2035-4a12-a777-98199a66beb8@proxel.se