depot_download_manager: fix verifying failed download

In some instances, the fetchurl progress determined by
depot_download_manager will be empty when the download fails (e.g. due
to a 404), this commit fixes a bug where this would be interpreted as
the download having completed which would subsequently start hash
verification of a package that has never actually been downloaded.

Fixes #4919

repos/gems/src/app/depot_download_manager/import.h

@@ -44,6 +44,10 @@ class Depot_download_manager::Import
 
 				bool complete() const
 				{
+					/* fetchurl did not return valid download info */
+					if (total == "")
+						return false;
+
 					/* fetchurl has not yet determined the file size */
 					if (total == "0.0")
 						return false;