outsmart gcc -O3 over-optimization in pkgCdrom::FindPackages

Seems like a simpler workaround than forcing a lower optimization
level just for this for all of apt.

See also:
https://bugs.launchpad.net/ubuntu/+source/gcc-5/+bug/1473674

apt-pkg/cdrom.cc

@@ -61,10 +61,10 @@ bool pkgCdrom::FindPackages(string CD,
       return _error->Errno("chdir","Unable to change to %s",CD.c_str());
 
    // Look for a .disk subdirectory
-   if (DirectoryExists(".disk") == true)
+   if (InfoDir.empty() == true)
    {
-      if (InfoDir.empty() == true)
-	 InfoDir = CD + ".disk/";
+      if (DirectoryExists(".disk") == true)
+	 InfoDir = InfoDir + CD + ".disk/";
    }
 
    // Don't look into directories that have been marked to ingore.

test/libapt/cdromfindpackages_test.cc

@@ -24,7 +24,9 @@ class Cdrom : public pkgCdrom {
 	    std::string &InfoDir) {
 	 std::string const startdir = SafeGetCWD();
 	 EXPECT_FALSE(startdir.empty());
+	 EXPECT_TRUE(InfoDir.empty());
 	 bool const result = pkgCdrom::FindPackages(CD, List, SList, SigList, TransList, InfoDir, NULL, 0);
+	 EXPECT_FALSE(InfoDir.empty());
 	 std::sort(List.begin(), List.end());
 	 std::sort(SList.begin(), SList.end());
 	 std::sort(SigList.begin(), SigList.end());