op.c: Make slabs sizes powers of two

It wasn’t exactly doubling the size of the previous slab, but making
it two less than that.  I’m assuming that malloc implementations round
things up to powers of two, and trying to take advantage of that, so
we don’t have wasted gaps at the ends of slabs.

op.c

@@ -222,9 +222,10 @@ Perl_Slab_Alloc(pTHX_ size_t sz)
 	/* Create a new slab.  Make this one twice as big. */
 	slot = slab2->opslab_first;
 	while (slot->opslot_next) slot = slot->opslot_next;
-	slab2 = S_new_slab(aTHX_ DIFF(slab2, slot)*2 > PERL_MAX_SLAB_SIZE
+	slab2 = S_new_slab(aTHX_
+			    (DIFF(slab2, slot)+1)*2 > PERL_MAX_SLAB_SIZE
 					? PERL_MAX_SLAB_SIZE
-					: DIFF(slab2, slot)*2);
+					: (DIFF(slab2, slot)+1)*2);
 	slab2->opslab_next = slab->opslab_next;
 	slab->opslab_next = slab2;
     }