Debugger: Initialize DumperBase.output

This was used uninitialized in some put call from lldbridge's
disassembler.

Change-Id: I5a3ecf075dfd4a6c4e8a483f9f8172e295329ad0
Reviewed-by: David Schulz <david.schulz@qt.io>

share/qtcreator/debugger/dumper.py

@@ -188,6 +188,7 @@ class DumperBase():
         self.displayStringLimit = 100
         self.useTimeStamps = False
 
+        self.output = ''
         self.typesReported = {}
         self.typesToReport = {}
         self.qtNamespaceToReport = None