Fix #34783: smoke simulation crash when changing frame while preview rendering.

Added a mutex lock for smoke data access. The render was already working with a
copy of the volume data, so it's just a short lock to copy things and should not
block the UI much.

source/blender/blenloader/intern/readfile.c

@@ -107,6 +107,7 @@
 #include "BLI_blenlib.h"
 #include "BLI_math.h"
 #include "BLI_edgehash.h"
+#include "BLI_threads.h"
 
 #include "BLF_translation.h"
 
@@ -4595,6 +4596,7 @@ static void direct_link_modifiers(FileData *fd, ListBase *lb)
 				smd->domain->smd = smd;
 				
 				smd->domain->fluid = NULL;
+				smd->domain->fluid_mutex = BLI_rw_mutex_alloc();
 				smd->domain->wt = NULL;
 				smd->domain->shadow = NULL;
 				smd->domain->tex = NULL;