[analyzer] Protect against dereferencing a null pointer

[alexander-shaposhnikov]

Summary

The check (inside StackHintGeneratorForSymbol::getMessage)

if (!N)
    return getMessageForSymbolNotFound()

is moved to the beginning of the function.

[NoQ]

Neat. I wonder if you've actually found it by running the analyzer :) Did it crash or just made sense to fix, i.e. should there be tests?

[alexander-shaposhnikov]

no, i have not seen any crashes related to this particular case, but i've been working on a fix for a separate issue (a bit more involved) (not far from here) (I will send a diff for review soon)
and decided to clean it up as well.

cfe/trunk/lib/StaticAnalyzer/Core/PathDiagnostic.cpp

for (PathPieces::const_iterator I = path.begin(), E = path.end(); I != E; ++I)
     ID.Add(**I);
   for (meta_iterator I = meta_begin(), E = meta_end(); I != E; ++I)
     ID.AddString(*I);
 }
 
 StackHintGenerator::~StackHintGenerator() {}
 
 std::string StackHintGeneratorForSymbol::getMessage(const ExplodedNode *N){
+  if (!N)
+    return getMessageForSymbolNotFound();
+
   ProgramPoint P = N->getLocation();
   CallExitEnd CExit = P.castAs<CallExitEnd>();
 
   // FIXME: Use CallEvent to abstract this over all calls.
   const Stmt *CallSite = CExit.getCalleeContext()->getCallSite();
   const CallExpr *CE = dyn_cast_or_null<CallExpr>(CallSite);
   if (!CE)
     return "";
 
-  if (!N)
-    return getMessageForSymbolNotFound();
-
   // Check if one of the parameters are set to the interesting symbol.
   unsigned ArgIndex = 0;
   for (CallExpr::const_arg_iterator I = CE->arg_begin(),
                                     E = CE->arg_end(); I != E; ++I, ++ArgIndex){
     SVal SV = N->getSVal(*I);
 
     // Check if the variable corresponding to the symbol is passed by value.
     SymbolRef AS = SV.getAsLocSymbol();