refine the characterization and fix for the BorrowShuffleBuffer() crash

VERSIONS

@@ -6,7 +6,7 @@ Note that not every commit will be logged here; that is what the Github commit h
 
 
 development head (in the master branch):
-	fix a crash when BorrowShuffleBuffer() is called with size zero, unfortunately introduced when debugging a different problem; occurs when a nonWF model has a subpop of size 0 at reproduction() time
+	fix a crash in BorrowShuffleBuffer(), introduced debugging a different problem; occurs when (a) a nonWF model has (b) a subpop of size 0 (c) at reproduction() time (d) before ever having a non-zero size
 
 
 version 4.2 (Eidos version 3.2):

core/species.cpp

@@ -3118,8 +3118,8 @@ slim_popsize_t *Species::BorrowShuffleBuffer(slim_popsize_t p_buffer_size)
 		EIDOS_TERMINATION << "ERROR (Species::BorrowShuffleBuffer): (internal error) shuffle buffer already borrowed." << EidosTerminate();
 
 #if DEBUG_SHUFFLE_BUFFER
-	// guarantee allocation even with a p_buffer_size of 0, so we have a place to put our overrun barriers
-	if ((p_buffer_size > shuffle_buf_capacity_) || (p_buffer_size == 0))
+	// guarantee allocation of a buffer, even with a requested size of 0, so we have a place to put our overrun barriers
+	if ((p_buffer_size > shuffle_buf_capacity_) || !shuffle_buffer_)
 #else
 	if (p_buffer_size > shuffle_buf_capacity_)
 #endif