download: Check for empty filename

Avoid an unhandled std::out_of_range exception if the client provides
an empty filename.

Change-Id: Ibfc99eee9adada255052375581bb308f7afa9da4
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/download_manager.cpp b/download_manager.cpp
index d600cf4..e6f7728 100644
--- a/download_manager.cpp
+++ b/download_manager.cpp
@@ -27,9 +27,12 @@
 {
 
     // Sanitize the fileName string
-    fileName.erase(std::remove(fileName.begin(), fileName.end(), '/'),
-                   fileName.end());
-    fileName = fileName.substr(fileName.find_first_not_of('.'));
+    if (!fileName.empty())
+    {
+        fileName.erase(std::remove(fileName.begin(), fileName.end(), '/'),
+                       fileName.end());
+        fileName = fileName.substr(fileName.find_first_not_of('.'));
+    }
 
     if (fileName.empty())
     {