Add missing GetImage method

Apparently, the analysis jobs require this method. It returns fImage that is always 0.

TJAlienFile.cxx

@@ -37,6 +37,7 @@ TJAlienFile::TJAlienFile(const char *purl, Option_t *option,
     fUrl = "";
     fPfn = "";
     fSE = "";
+    fImage = 0;
     fNreplicas = 0;
     fOpenedAt = time(0);
 }