[Server] Fix uncontrolled format string in XrdXrootdTpcMon

The format string in urlFMT is built from strings which come from
the environment, so one could pass in malicious strings that would
alter the actual format being used in XrdXrootdTpcMon::getURL().

src/XrdXrootd/XrdXrootdTpcMon.cc

@@ -46,7 +46,7 @@ const char *json_fmt = "{\"TPC\":\"%s\",\"Client\":\"%s\","
         "\"IPv\":%c},"
 "\"Src\":\"%s\",\"Dst\":\"%s\",\"Size\":%zu}";
 
-const char *urlFMT = "";
+const char *hostport = "";
 
 XrdSysError eDest(0, "Ouc");
 }
@@ -72,8 +72,8 @@ XrdXrootdTpcMon::XrdXrootdTpcMon(const char       *proto,
    const char *host = getenv("XRDHOST"); if (!host) host = "localhost";
    const char *port = getenv("XRDPORT"); if (!port) {colon = ""; port = "";}
 
-   snprintf(buff, sizeof(buff), "%%s://%s%s%s/%%s", host, colon, port);
-   urlFMT = strdup(buff);
+   snprintf(buff, sizeof(buff), "%s%s%s", host, colon, port);
+   hostport = strdup(buff);
 }
 
 /******************************************************************************/
@@ -86,7 +86,7 @@ const char *XrdXrootdTpcMon::getURL(const char *spec, const char *prot,
 // Handle the spec
 //
    if (*spec == '/')
-      {snprintf(buff, bsz, urlFMT, prot, spec);
+      {snprintf(buff, bsz, "%s://%s/%s", prot, hostport, spec);
        spec = buff;
       }
    return spec;