fix crash with population size 0 in some cases

VERSIONS

@@ -6,7 +6,8 @@ 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
+
 
 version 4.2 (Eidos version 3.2):
 	fix for #418, a crash involving null genomes in nonWF (has_null_genomes_ was not set correctly by addCloned() or takeMigrants() when putting a null genome into a subpop that previously had none)

core/species.cpp

@@ -3117,7 +3117,12 @@ slim_popsize_t *Species::BorrowShuffleBuffer(slim_popsize_t p_buffer_size)
 	if (shuffle_buf_borrowed_)
 		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))
+#else
 	if (p_buffer_size > shuffle_buf_capacity_)
+#endif
 	{
 		if (shuffle_buffer_)
 			free(shuffle_buffer_);