Fix root-project#11383 (deadlock at initialization) (root-project#11384)

Previously if a message (for example warning of duplicate dictionary)
happened during intialization, it would trigger the initialization of gROOT
and could trigger a nested Warning. for example due to the duplicate rootmap
file entry and would cause a dead lock (recursiverly taking the
non recursive lock GetErrorMutex().

core/base/src/TErrorDefaultHandler.cxx

@@ -101,12 +101,14 @@ static void DebugPrint(const char *fmt, ...)
 void DefaultErrorHandler(Int_t level, Bool_t abort_bool, const char *location, const char *msg)
 {
    if (gErrorIgnoreLevel == kUnset) {
-      std::lock_guard<std::mutex> guard(*GetErrorMutex());
-
-      gErrorIgnoreLevel = 0;
       if (gEnv) {
-         std::string slevel;
+         // This can also print error messages, so we need to do it outside the lock
          auto cstrlevel = gEnv->GetValue("Root.ErrorIgnoreLevel", "Print");
+
+         std::lock_guard<std::mutex> guard(*GetErrorMutex());
+
+         gErrorIgnoreLevel = 0;
+         std::string slevel;
          while (cstrlevel && *cstrlevel) {
             slevel.push_back(tolower(*cstrlevel));
             cstrlevel++;