Fix an obscure warning from GCC

Fix an obscure warning from GCC

It complained about the previous code:

apt-pkg/sourcelist.cc: In destructor ‘pkgSourceList::~pkgSourceList()’:
apt-pkg/sourcelist.cc:278:4: warning: cannot optimize loop, the loop counter may overflow [-Wunsafe-loop-optimizations]
    for (pkgIndexFile * const File : VolatileFiles)
    ^
There really cannot be an overflow, though. Rewriting it like this
seems to fix it.