stapbpf (for PR27030): bugfix the b71d20af bugfix

stapbpf (for PR27030): bugfix the b71d20af bugfix

stap commit b71d20af819 fixed error messages in bpf assembly
(being broken by implicit deallocation of stack vars on exception throw)
but neglected the fact that visit_embeddedcode can recurse
(via emit_functioncall) such that a single bpf_unparser field for
storing asm_stmts will get overwritten by the recursive call.

Alloc/free a separate statement list per visit_embeddedcode call instead.