Bug 290247: fix 3 long-standing do_CreateInstanceFromCategory bugs:

- initialize mOuter so we don't always try to aggregate against a random
  stack pointer;
- don't try to store through the oft-null mErrorPtr
- set rv so that we actually check CreateInstanceByContractID's return
r=bsmedberg, sr=darin, a=brendan

xpcom/components/nsCategoryManagerUtils.h

@@ -53,6 +53,7 @@ public:
                                  nsISupports *aOuter, nsresult *aErrorPtr)
         : mCategory(aCategory),
           mEntry(aEntry),
+          mOuter(aOuter),
           mErrorPtr(aErrorPtr)
     {
         // nothing else to do;

xpcom/components/nsComponentManager.cpp

@@ -197,16 +197,14 @@ nsCreateInstanceFromCategory::operator()(const nsIID& aIID, void** aInstancePtr)
     NS_GetComponentManager(getter_AddRefs(compMgr));
     if (!compMgr)
         return NS_ERROR_FAILURE;
-    compMgr->CreateInstanceByContractID(value,
-                                        mOuter,
-                                        aIID,
-                                        aInstancePtr);
+    rv = compMgr->CreateInstanceByContractID(value, mOuter, aIID, aInstancePtr);
     if (NS_FAILED(rv)) {
     error:
         *aInstancePtr = 0;
     }
 
-    *mErrorPtr = rv;
+    if (mErrorPtr)
+        *mErrorPtr = rv;
     return rv;
 }