libdwfl: Declare possible zero sized arrays only when non-zero

libdwfl: Declare possible zero sized arrays only when non-zero

The gcc undefined sanitizer complains when seeing a zero sized array
declaration. Move the declaration to the point in the code where we
know they aren't zero sized.

https://sourceware.org/bugzilla/show_bug.cgi?id=28720

Signed-off-by: Mark Wielaard <mark@klomp.org>