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.
16 jobs for next in 238 minutes and 51 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Build
passed #143672
alpine-gcc

00:53:37

passed #143673
arch-clang

00:49:16

passed #143674
arch-gcc-glibcxxdebug

01:16:06

passed #143671
debian-gcc-snapshot

01:11:19

passed #143669
debian-stable-gcc

01:00:47

passed #143670
debian-unstable-gcc-coverage

00:34:47

90.7%
passed #143677
debpkg-stable

01:21:59

passed #143678
debpkg-unstable

01:11:47

passed #143675
mingw-shared

00:12:05

passed #143676
mingw-static

00:09:47

passed #143680
armv7
raspbian

01:30:53

passed #143679
rpm-pkg

01:06:56

 
  Build2
passed #143681
x86
debpkg-stable-i386

01:14:58

passed #143682
x86
debpkg-unstable-i386

00:58:28

 
  Publish
passed #143683
dput
publish-rpm

00:00:47

passed #143684
dput
publish-unstable

00:03:21