Fix crashes in apt search, cache file

When BuildDepCaches() is called before BuildCaches() we end up
with dereferencing a null pointer cache in the depcache constructor.

Furthermore, in private-search, we check that Cache == NULL, but
only after constructing the policy and records, which does not
work reliably, because the records construction accesses the cache
as well.

LP: #1815187
Test-Case: ./build/bin/apt search -o RootDir=/dev/null foo

1 files changed, 5 insertions, 1 deletions

diff --git a/apt-private/private-search.cc b/apt-private/private-search.cc
index ecd5d7fad..e218bf76c 100644
--- a/apt-private/private-search.cc
+++ b/apt-private/private-search.cc
@@ -31,9 +31,13 @@ bool FullTextSearch(CommandLine &CmdL)					/*{{{*/
 {
    pkgCacheFile CacheFile;
    pkgCache *Cache = CacheFile.GetPkgCache();
+   if (unlikely(Cache == NULL))
+      return false;
+
    pkgDepCache::Policy *Plcy = CacheFile.GetPolicy();
    pkgRecords records(CacheFile);
-   if (unlikely(Cache == NULL || Plcy == NULL))
+
+   if (unlikely(Plcy == NULL))
       return false;
 
    const char **patterns;