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.c

@@ -328,7 +328,7 @@ initscan(HeapScanDesc scan, ScanKey key, bool keep_startblock)
 	/*
 	 * copy the scan key, if appropriate
 	 */
-	if (key != NULL)
+	if (key != NULL && scan->rs_base.rs_nkeys > 0)
 		memcpy(scan->rs_base.rs_key, key, scan->rs_base.rs_nkeys * sizeof(ScanKeyData));
 
 	/*