elf: Fix SIGSEGV if dynamic table section is missing

If an ELF object does not have a dynamic table section, throw a proper
exception from _dynamic_tag() instead of faulting on nullptr.

Spotted while trying to run a static ELF binary.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>