Make it so if xmalloc returns NULL we still get a size in the perl api.

This changed in 17.11 where if the size was 0 we would return 0 which messes
up the perl api.

Bug 3644

NEWS

@@ -13,6 +13,7 @@ documents those changes that are of interest to users and administrators.
  -- When running the "scontrol top" command, make sure that all of the user's
     jobs have a priority that is lower than the selected job. Previous logic
     would permit other jobs with equal priority (no jobs with higher priority).
+ -- Fix perl api so we always get an allocation when calling Slurm::new().
 
 * Changes in Slurm 17.02.2
 ==========================

contribs/perlapi/libslurm/perl/Slurm.xs

@@ -30,7 +30,12 @@ static struct slurm default_slurm_object;
 static slurm_t
 new_slurm(void)
 {
-	return xmalloc(sizeof(struct slurm));
+	int size = sizeof(struct slurm);
+	if (size == 0) {
+		/* Avoid returning NULL, which causes the perl APIs to fail */
+		size = 1;
+	}
+	return xmalloc(size);
 }
 
 static void