selftests/mm: fix potential wild pointer access of getline due to missing init

selftests/mm: fix potential wild pointer access of getline due to missing init

This is another occurrence of using getline where the code assumes that
getline allocates memory to store the line, but the pointer passed to it
is uninitialized and potentially a non-null pointer.  This violates the
Open Group Spec[1] and caused a segfault in a similar situation in
selftest/clone3/clone3_set_tid.  Fix it by initializing the line pointer
to NULL.

The issue has been found by simply grepping through the selftest code
after running into the issue in clone3_set_tid.  Whether it segfaults in
its current state is unknown to me.  But it's good to be addressed due to
defensive reasons.

Link: https://lore.kernel.org/20260722130246.2135563-3-christian.gellermann@codasip.com
Link: https://pubs.opengroup.org/onlinepubs/9799919799/functions/getline.html
Fixes: 26b4224d9961 ("selftests: expanding more mlock selftest")
Signed-off-by: Chris Gellermann <christian.gellermann@codasip.com>
Acked-by: David Hildenbrand (arm) <david@kernel.org>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>