stapbpf (for PR27030): bugfix error messages in bpf assembly

stapbpf (for PR27030): bugfix error messages in bpf assembly

Need to retain the asm_stmts vector so that bpf assembly tokens
are not deallocated on exception throw.

Otherwise, printing semantic errors from bpf assembly causes segfault.

* bpf-translate.cxx (struct bpf_unparser): retain asm_stmts vector.
(bpf_unparser::visit_embeddedcode): reuse retained asm_stmts vector.