Fix std::out_of_range exception with short filenames (Linux)

src/coreclr/hosts/unixcorerun/corerun.cpp

@@ -190,7 +190,7 @@ void AddFilesFromDirectoryToTpaList(const char* directory, std::string& tpaList)
             std::string filename(entry->d_name);
 
             // Check if the extension matches the one we are looking for
-            size_t extPos = filename.length() - extLength;
+            int extPos = filename.length() - extLength;
             if ((extPos <= 0) || (filename.compare(extPos, extLength, ext) != 0))
             {
                 continue;