fix 'apt-cache search' crash with missing description

Beside the earlier fixed 'apt-cache show', 'showpkg' and 'search' deal
with descriptions. 'showpkg' was fixed by fixing the cache generation
for 'show', but 'search' still segfaulted.

On the upside, it doesn't segfault any longer, on the downside, if a
package has no description at all (aka: not in the Packages file and not
in a Translation-* file) the package can't be found with 'search', even
if we search only by name. That is a shortcoming in the code, but fixing
it means rewriting it completely for dubious gain at best.

So this commit just skips packages without a description and is done.

Closes: 647590