tar: fix bug when -s/a/b/ used more than once with b flag (#2435)

tar: fix bug when -s/a/b/ used more than once with b flag (#2435)

When the -s/regexp/replacement/ option was used with the b flag more
than once, the result of the previous substitution was appended to the
previous subject instead of replacing it. Fixed it by making sure the
subject is made the empty string before the call to realloc_strcat().
That in effect makes it more like a realloc_strcpy(), but creating a new
realloc_strcpy() function for that one usage doesn't feel worth it.

Resolves Issue libarchive/libarchive#2414

Co-authored-by: Stephane Chazelas <stephane@chazelas.org>