find: Fix segfault with very long path in -exec/-ok ... {} \;.

If the resulting argument is longer than MAXPATHLEN, realloc() was called to
extend the space, but the new pointer was not correctly stored.

Different from what OpenBSD has done, rewrite brace_subst() to calculate the
necessary space first and realloc() at most once.

As before, the e_len fields are not updated in case of a realloc.
Therefore, a following long argument will do another realloc.

PR: 201750
MFC after: 1 week