Debugger: Fix GDB hanging

This partially reverts commit e33da6a6b3.
GDB seems to hang while executing str(some_pointer) in some setups.

Task-number: QTCREATORBUG-14374
Change-Id: Ia678533b9122e505b21ef6659d404239995ddb81
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Christian Stenger <christian.stenger@theqtcompany.com>

share/qtcreator/debugger/gdbbridge.py

@@ -522,6 +522,14 @@ class Dumper(DumperBase):
         except:
             return None
 
+    def isBadPointer(self, value):
+        try:
+            target = value.dereference()
+            target.is_optimized_out # Access test.
+            return False
+        except:
+            return True
+
     def makeValue(self, typeobj, init):
         typename = "::" + self.stripClassTag(str(typeobj));
         # Avoid malloc symbol clash with QVector.