port valgrind fix from Neil Wilson

Author: rkubik

src/cc/frontends/clang/b_frontend_action.cc

@@ -321,7 +321,7 @@ bool MapVisitor::VisitCallExpr(CallExpr *Call) {
 
 ProbeVisitor::ProbeVisitor(ASTContext &C, Rewriter &rewriter,
                            set<Decl *> &m, bool track_helpers) :
-  C(C), rewriter_(rewriter), m_(m), track_helpers_(track_helpers),
+  C(C), rewriter_(rewriter), m_(m), ctx_(nullptr), track_helpers_(track_helpers),
   addrof_stmt_(nullptr), is_addrof_(false) {
   const char **calling_conv_regs = get_call_conv();
   has_overlap_kuaddr_ = calling_conv_regs == calling_conv_regs_s390x;