ndr: avoid excessive reallocing in pull_string_array

ndr: avoid excessive reallocing in pull_string_array

Before, talloc_realloc() was being called n times for an array of
length n. This could be very expensive on long string arrays since it
is reasonable to assume each realloc moves O(n) bytes.

This addresses at least one OSS-Fuzz bug, making a timing out test case
100 times faster. Credit to OSS-Fuzz.

REF: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=19706

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Noel Power <npower@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>