canonicalize: avoid uninitialized memory use

When DOUBLE_SLASH_IS_DISTINCT_ROOT is non-zero, then we were
reading the contents of rpath[1] even when we had never written
anything there, which meant that "///" would usually canonicalize
to "/" but sometimes to "//" if a '/' was leftover in the heap.
This condition could also occur via 'ln -s / //some/path' and
canonicalizing //some/path, where we rewind rpath but do not
clear out the previous round.  Platforms where "//" and "/" are
equivalent do not suffer from this read-beyond-written bounds.

* lib/canonicalize-lgpl.c (__realpath): Avoid possibility of
random '/' left in dest.
* lib/canonicalize.c (canonicalize_filename_mode): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>

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