elf: Fix alloca size in _dl_debug_vdprintf

The alloca size did not consider the optional width parameter for
padding which could cause buffer underflow. The width is currently used
e.g. by _dl_map_object_from_fd which passes 2 * sizeof(void *) which
can be larger than the alloca buffer size on targets where
sizeof(void *) >= 2 * sizeof(unsigned long).

Even if large width is not used on existing targets it is better to fix
the formatting code to avoid surprises.

Reviewed-by: Florian Weimer <fweimer@redhat.com>