Fixed EXT-6423(normal) - Windows viewer spends a minute clearing cache when there is no cache

Problem description:
The code that purges cache does not expect any folders there, error occurs when it tries to delete a file, which is actually a folder. To handle an error the code sleeps for 1 second and tries to delete the file 4 more times(sleeping for 1 second after each try).
Fix info:
Using LLFile::isDir to skip directories and only delete files.

Reviewed by Vadim Savchuk - https://codereview.productengine.com/secondlife/r/239/

--HG--
branch : product-engine

indra/llvfs/lldir.cpp

@@ -91,15 +91,16 @@ S32 LLDir::deleteFilesInDir(const std::string &dirname, const std::string &mask)
 	S32 result;
 	while (getNextFileInDir(dirname, mask, filename, FALSE))
 	{
-		if ((filename == ".") || (filename == ".."))
+		fullpath = dirname;
+		fullpath += getDirDelimiter();
+		fullpath += filename;
+
+		if(LLFile::isdir(fullpath))
 		{
 			// skipping directory traversal filenames
 			count++;
 			continue;
 		}
-		fullpath = dirname;
-		fullpath += getDirDelimiter();
-		fullpath += filename;
 
 		S32 retry_count = 0;
 		while (retry_count < 5)