Floor disk_count at zero.

server/block_processor.py

@@ -828,7 +828,7 @@ class BlockProcessor(server.db.DB):
 
     def read_headers(self, start, count):
         # Read some from disk
-        disk_count = min(count, self.fs_height + 1 - start)
+        disk_count = min(count, max(0, self.fs_height + 1 - start))
         result = self.fs_read_headers(start, disk_count)
         count -= disk_count
         start += disk_count

server/db.py

@@ -143,6 +143,7 @@ class DB(LoggedClass):
             raise
 
     def fs_read_headers(self, start, count):
+        '''Requires count >= 0.'''
         # Read some from disk
         disk_count = min(count, self.db_height + 1 - start)
         if start < 0 or count < 0 or disk_count != count: