QMLDebugging: Fix crash

Check for null pointers when parsing AST to relocate breakpoint

Change-Id: I484048d9e8a7aabdeffc4e18049c9b9b6f030366
Reviewed-by: Simjees Abraham <simjees.abraham@nokia.com>
Reviewed-by: Kai Koehne <kai.koehne@nokia.com>
Reviewed-by: Roberto Raggi <roberto.raggi@nokia.com>

src/plugins/debugger/qml/qmlengine.cpp

@@ -154,6 +154,9 @@ public:
 
     bool visit(UiScriptBinding *ast)
     {
+        if (!ast->statement)
+            return true;
+
         quint32 sourceStartLine = ast->firstSourceLocation().startLine;
         quint32 statementStartLine;
         quint32 statementColumn;
@@ -165,6 +168,8 @@ public:
 
         } else if (ast->statement->kind == Node::Kind_Block) {
             Block *block = static_cast<Block *>(ast->statement);
+            if (!block || !block->statements)
+                return true;
             statementStartLine = block->statements->firstSourceLocation().
                     startLine;
             statementColumn = block->statements->firstSourceLocation().