Fix assertion in symbol library remap dialog.

Do not attempt to normalize environment variables that are not valid
paths, paths that do not exist, or paths that the user does not have
read access to.

Fixes lp:1731655

https://bugs.launchpad.net/kicad/+bug/1731655

eeschema/dialogs/dialog_symbol_remap.cpp

@@ -136,6 +136,11 @@ void DIALOG_SYMBOL_REMAP::createProjectSymbolLibTable( REPORTER& aReporter )
 
             for( auto& entry : envMap )
             {
+                // Don't bother normalizing paths that don't exist or the user cannot read.
+                if( !wxFileName::DirExists( entry.second.GetValue() )
+                  || !wxFileName::IsDirReadable( entry.second.GetValue() ) )
+                    continue;
+
                 envPath.SetPath( entry.second.GetValue() );
 
                 if( normalizeAbsolutePaths( envPath, fn, &tmp ) )