Bug 708629. Avoid using uninitialized pkey_stack. r=bgirard

This was causing crashes with make check/xpcshell on OS X.

tools/profiler/sps/TableTicker.cpp

@@ -52,6 +52,11 @@ using std::string;
 
 pthread_key_t pkey_stack;
 pthread_key_t pkey_ticker;
+// We need to track whether we've been initialized otherwise
+// we end up using pkey_stack without initializing it.
+// Because pkey_stack is totally opaque to us we can't reuse
+// it as the flag itself.
+bool stack_key_initialized;
 
 TimeStamp sLastTracerEvent;
 
@@ -378,6 +383,7 @@ void mozilla_sampler_init()
     LOG("Failed to init.");
     return;
   }
+  stack_key_initialized = true;
 
   Stack *stack = new Stack();
   pthread_setspecific(pkey_stack, stack);

tools/profiler/sps/sps_sampler.h

@@ -47,6 +47,7 @@ using mozilla::TimeDuration;
 // TODO Merge into Sampler.h
 
 extern pthread_key_t pkey_stack;
+extern bool stack_key_initialized;
 
 #define SAMPLER_INIT() mozilla_sampler_init();
 #define SAMPLER_DEINIT() mozilla_sampler_deinit();
@@ -194,7 +195,16 @@ public:
 
 inline void* mozilla_sampler_call_enter(const char *aInfo)
 {
+  // check if we've been initialized to avoid calling pthread_getspecific
+  // with a null pkey_stack which will return undefined results.
+  if (!stack_key_initialized)
+    return NULL;
+
   Stack *stack = (Stack*)pthread_getspecific(pkey_stack);
+  // we can't infer whether 'stack' has been initialized
+  // based on the value of stack_key_intiailized because
+  // 'stack' is only intialized when a thread is being
+  // profiled.
   if (!stack) {
     return stack;
   }