AST: leave and scope OPs are done
Make pj_build_block_or_term more robust

To wit before:
perl author_tools/jit_ast_dump.pl -c -e '$x = do {1;1;1}'
----------------------------------------------
 StatementSequence
   Statement (eval 13):1
     Binop 'sassign' (
       G = ($)
       Block 'scope' (
         StatementSequence
           Statement (eval 13):1
             'UnJITable subtree'
       )
    )
----------------------------------------------
B::Concise::compile(CODE(0x21eafd8))
9  <1> leavesub[1 ref] K/REFC,1 ->(end)
-     <@> lineseq KP ->9
1        <;> nextstate(main 3 (eval 13):1) v:%,{,2048 ->2
8        <2> sassign sKS/2 ->9
-           <1> null sK*/1 ->7
6              <@> leave sKP ->7
2                 <0> enter s ->3
3                 <;> nextstate(main 2 (eval 13):1) v:%,2048 ->4
-                 <0> ex-const v ->-
-                 <0> null v ->-
-                 <0> ex-const v ->4
4                 <;> nextstate(main 2 (eval 13):1) v:%,{,2048 ->5
5                 <$> const[IV 1] s ->6
-           <1> ex-rv2sv sKRM*/1 ->8
7              <#> gvsv[*x] s ->8

And after:
----------------------------------------------
 StatementSequence
   Statement (eval 13):1
     Binop 'sassign' (
       G = ($)
       Block 'scope' (
         StatementSequence
           Statement (eval 13):1
             C = (IV)1
       )
     )
----------------------------------------------
B::Concise::compile(CODE(0xc35fd8))
9  <1> leavesub[1 ref] K/REFC,1 ->(end)
-     <@> lineseq KP ->9
1        <;> nextstate(main 3 (eval 13):1) v:%,{,2048 ->2
8        <2> sassign sKS/2 ->9
-           <1> null sK*/1 ->7
6              <@> leave sKP ->7
2                 <0> enter s ->3
3                 <;> nextstate(main 2 (eval 13):1) v:%,2048 ->4
-                 <0> ex-const v ->-
-                 <0> null v ->-
-                 <0> ex-const v ->4
4                 <;> nextstate(main 2 (eval 13):1) v:%,{,2048 ->5
5                 <$> const[IV 1] s ->6
-           <1> ex-rv2sv sKRM*/1 ->8
7              <#> gvsv[*x] s ->8