[EVAL] Avoid using undefined handler

* src/eval.c (evalbltin, evalfun): Set savehandler before calling
setjmp with the possible "goto *done", where savehandler is used.
Otherwise, clang warns that "Assigned value is garbage or undefined"
at the point where "savehandler" is used on the RHS.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>