[PATCH setup] Avoid potential null deref in check_for_cached()

[Jon Turney]

Packages for which we don't know a filename can't possibly be cached.

See https://cygwin.com/ml/cygwin/2018-10/msg00033.html
---
 download.cc | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/download.cc b/download.cc
index 0cb3352..55c6295 100644
--- a/download.cc
+++ b/download.cc
@@ -84,9 +84,16 @@ int
 check_for_cached (packagesource & pkgsource, HWND owner, bool mirror_mode,
 		  bool check_hash)
 {
+  /* If the packagesource doesn't have a filename, it can't possibly be in the
+     cache */
+  if (!pkgsource.Canonical())
+    {
+      return 0;
+    }
+
   /* Note that the cache dir is represented by a mirror site of file://local_dir */
   std::string prefix = "file://" + local_dir + "/";
-  std::string fullname = prefix + (pkgsource.Canonical() ? pkgsource.Canonical() : "");
+  std::string fullname = prefix + pkgsource.Canonical();
 
   if (mirror_mode)
     {
-- 
2.17.0