fix(par): header receive counter was allocated to the wrong size (#1971)

Closes #1964

src/Distributed/MpiRouter.f90

@@ -402,7 +402,7 @@ subroutine compose_messages(this, unit, stage, body_snd_t, body_rcv_t)
     allocate (hdr_rcv_t(this%receivers%size))
     allocate (hdr_snd_t(this%senders%size))
     allocate (headers(max_headers, this%receivers%size))
-    allocate (hdr_rcv_cnt(max_headers))
+    allocate (hdr_rcv_cnt(this%receivers%size))
 
     ! allocate map data
     allocate (map_snd_t(this%senders%size))