QmakePM: Avoid soft assertion

... when a matching toolchain is not found.

Change-Id: I901bdb51f98a88b2402b12309a05e0bc5ffcaa3b
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/qmakeprojectmanager/qmakekitinformation.cpp

@@ -87,7 +87,8 @@ void QmakeKitInformation::setup(Kit *k)
                     break;
             }
         }
-        ToolChainKitInformation::setToolChain(k, possibleTc);
+        if (possibleTc)
+            ToolChainKitInformation::setToolChain(k, possibleTc);
     }
 }