Avoid useless allocations for information of dumpable objects in pg_dump/

If there are no objects of a certain type, there is no need to do an
allocation for a set of DumpableObject items.  The previous coding did
an allocation of 1 byte instead as per the fallback of pg_malloc() in
the event of an allocation size of zero.  This assigns NULL instead for
a set of dumpable objects.

A similar rule already applied to findObjectByOid(), so this makes the
code more defensive as we would just fail with a pointer dereference
instead of attempting to use some incorrect data if a non-existing,
positive, OID is given by a caller of this function.

Author: Daniel Gustafsson
Reviewed-by: Julien Rouhaud, Ranier Vilela
Discussion: https://postgr.es/m/26C43E58-BDD0-4F1A-97CC-4A07B52E32C5@yesql.se

Author: michaelpq

src/bin/pg_dump/common.c

@@ -719,6 +719,9 @@ buildIndexArray(void *objArray, int numObjs, Size objSize)
 	DumpableObject **ptrs;
 	int			i;
 
+	if (numObjs <= 0)
+		return NULL;
+
 	ptrs = (DumpableObject **) pg_malloc(numObjs * sizeof(DumpableObject *));
 	for (i = 0; i < numObjs; i++)
 		ptrs[i] = (DumpableObject *) ((char *) objArray + i * objSize);