selftests: coredump: Properly initialize pointer

selftests: coredump: Properly initialize pointer

The buffer pointer "line" is not initialized. This pointer is passed to
getline().

It can still work if the stack is zero-initialized, because getline() can
work with a NULL pointer as buffer.

But this is obviously broken. This bug shows up while running the test on a
riscv64 machine.

Fix it by properly initializing the pointer.

Fixes: 15858da53542 ("selftests: coredump: Add stackdump test")
Signed-off-by: Nam Cao <namcao@linutronix.de>
Link: https://lore.kernel.org/4fb9b6fb3e0040481bacc258c44b4aab5c4df35d.1744383419.git.namcao@linutronix.de
Signed-off-by: Christian Brauner <brauner@kernel.org>