Fix ldconfig segmentation fault with corrupted cache (Bug 18093).

ldconfig is using an aux-cache to speed up the ld.so.cache update. It
is read by mmaping the file to a structure which contains data offsets
used as pointers. As they are not checked, it is not hard to get
ldconfig to segfault with a corrupted file. This happens for instance if
the file is truncated, which is common following a filesystem check
following a system crash.

This can be reproduced for example by truncating the file to roughly
half of it's size.

There is already some code in elf/cache.c (load_aux_cache) to check
for a corrupted aux cache, but it happens to be broken and not enough.
The test (aux_cache->nlibs >= aux_cache_size) compares the number of
libs entry with the cache size. It's a non sense, as it basically
assumes that each library entry is a 1 byte... Instead this commit
computes the theoretical cache size using the headers and compares it
to the real size.