work around a null pointer dereference error

This fixes #462.

* spot/twaalgos/gfguarantee.cc (do_g_f_terminal_inplace): Replace the
redirect_src vector by a unique_ptr<unsigned[]>.  Not only does this
remove the false positive diagnostic, but it also removes the unneeded
default initialization of the elements of that vector.
13 jobs for adl/462 in 235 minutes and 41 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #143660
alpine-gcc

00:57:50

passed #143661
arch-clang

00:51:17

passed #143629
arch-gcc-glibcxxdebug

01:11:29

passed #143659
debian-gcc-snapshot

01:11:19

passed #143658
debian-stable-gcc

01:02:12

passed #143625
debian-unstable-gcc-coverage

00:34:19

90.7%
passed #143630
mingw-shared

00:12:33

passed #143631
mingw-static

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passed #143632
armv7
raspbian

01:31:37

failed #143627
alpine-gcc

00:47:31

failed #143628
arch-clang

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failed #143626
allowed to fail
debian-gcc-snapshot

00:47:31

failed #143624
debian-stable-gcc

00:47:37