CppTools: Add missing null pointer check

Prevents a crash trying to look for decl/def matches in non-applicable
locations.

Fixes: QTCREATORBUG-25806
Change-Id: I70961109267a2955bef0434a92f1d913b8c0c9c1
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/plugins/cpptools/symbolfinder.cpp

@@ -436,6 +436,9 @@ QList<Declaration *> SymbolFinder::findMatchingDeclaration(const LookupContext &
                                                            Function *functionType)
 {
     QList<Declaration *> result;
+    if (!functionType)
+        return result;
+
     QList<Declaration *> nameMatch, argumentCountMatch, typeMatch;
     findMatchingDeclaration(context, functionType, &typeMatch, &argumentCountMatch, &nameMatch);
     result.append(typeMatch);