Clean up assorted failures under clang's -fsanitize=undefined checks.

Most of these are cases where we could call memcpy() or other libc
functions with a NULL pointer and a zero count, which is forbidden
by POSIX even though every production version of libc allows it.
We've fixed such things before in a piecemeal way, but apparently
never made an effort to try to get them all.  I don't claim that
this patch does so either, but it gets every failure I observe in
check-world, using clang 12.0.1 on current RHEL8.

numeric.c has a different issue that the sanitizer doesn't like:
"ln(-1.0)" will compute log10(0) and then try to assign the
resulting -Inf to an integer variable.  We don't actually use the
result in such a case, so there's no live bug.

Back-patch to all supported branches, with the idea that we might
start running a buildfarm member that tests this case.  This includes
back-patching c1132aae3 (Check the size in COPY_POINTER_FIELD),
which previously silenced some of these issues in copyfuncs.c.

Discussion: https://postgr.es/m/CALNJ-vT9r0DSsAOw9OXVJFxLENoVS_68kJ5x0p44atoYH+H4dg@mail.gmail.com

src/backend/access/heap/heapam_visibility.c

@@ -1564,8 +1564,8 @@ HeapTupleHeaderIsOnlyLocked(HeapTupleHeader tuple)
 static bool
 TransactionIdInArray(TransactionId xid, TransactionId *xip, Size num)
 {
-	return bsearch(&xid, xip, num,
-				   sizeof(TransactionId), xidComparator) != NULL;
+	return num > 0 &&
+		bsearch(&xid, xip, num, sizeof(TransactionId), xidComparator) != NULL;
 }
 
 /*