Fix wrong type for main_bitmap

Fix IntelLabs/kAFL#298
This appears to happen when the target is slow and the manager tries to check for initial coverage? It can also happen when using multiple instances. The commit fixes the incorrect required type for `mm3.hash`.

kafl_fuzzer/manager/manager.py

@@ -78,7 +78,7 @@ def send_next_task(self, conn):
         self.busy_events +=1
         if self.busy_events >= self.config.processes:
             self.busy_events = 0
-            main_bitmap = self.bitmap_storage.get_bitmap_for_node_type("regular").c_bitmap
+            main_bitmap = bytes(self.bitmap_storage.get_bitmap_for_node_type("regular").c_bitmap)
             if mmh3.hash(main_bitmap) == self.empty_hash:
                 logger.warn("Coverage bitmap is empty?! Check -ip0 or try better seeds.")