ef_elt_by_name(), xdprval_sym() and symval() checked whether a file
type T is a symbol table by comparing ef_cadef(T) to symbol_ca, even
though T may be EF_BAD. Before commit 50cfdcb5, ef_cadef(EF_BAD)
accessed empfile[] out of bounds, which could conceivably crash or
somehow happen to yield symbol_ca. Since then, it oopses and returns
null.
Fix by testing the file type before calling ef_cadef().
This is for the forthcoming empdump utility program. The xdump
command still does not support the human-readable dialect.
xundump has supported the human-readable dialect for a long time
(commit 4871a10a).
