Fix off-by-one in TCO

Now we have the ability to define a generator in jq:

        def for(cond; update):
            def _for:
                if cond then ., (update | _for) else . end;
            _for;

        for(. < 10; . + 1) # generates numbers between `.` and 10

    Running this by hand with --debug-dump-disasm (with a fix for that
    coming up next) we can see that the call to _for is optimized:

        _for:0:
          0000 DUP
          0001 CALL_JQ cond:0^1
          0005 JUMP_F 0022
          0007 POP
          0008 FORK 0012
          0010 JUMP 0020
          0012 CALL_JQ update:1^1
          0016 TAIL_CALL_JQ _for:0^1
          0020 JUMP 0023
          0022 POP
          0023 RET

    And timing this with 1000, 10000, 100000 iterations shows that
    indeed we must be applying TCO; otherwise, without TCO, this gets
    very slow very quickly.

execute.c

@@ -801,7 +801,7 @@ static int ret_follows(uint16_t *pc) {
     return 1;
   if (*pc++ != JUMP)
     return 0;
-  return ret_follows(pc + *pc); // FIXME, might be ironic
+  return ret_follows(pc + *pc + 1); // FIXME, might be ironic
 }
 
 /*