Fix runtime errors from -fsanitize=undefined

After many years of trying, it seems the -fsanitize=undefined checking
in gcc is now working somewhat reliably.  Attached is a patch that fixes
all errors of the kind

runtime error: null pointer passed as argument N, which is declared to
never be null

Most of the cases are calls to memcpy(), memcmp(), etc. with a length of
zero, so one appears to get away with passing a null pointer.

Note that these are runtime errors, not static analysis, so the code in
question is actually reached.

To reproduce, configure normally and then set

COPT=-fsanitize=undefined -fno-sanitize=alignment -fno-sanitize-recover=all

and build and run make check-world.  Unpatched, this will core dump in
various places.

(-fno-sanitize=alignment should also be fixed but I took it out here to
deal with it separately.)

See https://gcc.gnu.org/onlinedocs/gcc/Instrumentation-Options.html for
further documentation.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
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