Fix misuse of ef_cadef(EF_BAD)

Fix misuse of ef_cadef(EF_BAD)

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().