avoid next_node being forced

Author: tybug

hypothesis-python/src/hypothesis/internal/conjecture/shrinker.py

@@ -1270,6 +1270,12 @@ def redistribute_block_pairs(self, chooser):
         else:
             return
 
+        if next_node.was_forced:
+            # avoid modifying a forced node! Note that it's fine for next_node
+            # to be trivial, because we're going to explicitly make it *not*
+            # trivial by adding to its value.
+            return
+
         m = node.value
         n = next_node.value