Runtime: fix incorrect pos_in after unmarshall

[hhugo]

should fix #1668
@ejgallego can you confirm ?

[ejgallego]

Thanks a lot @hhugo , this makes the Marshall error disappear, however there seems to be some other issue with 4.1.0 and dynlink (investigating that); but this patch seems to be good for the first problem.

[ejgallego]

Did some more testing on 4.1.0-dev + this patch. I have found that:

• 9b82bd9 "bump magic number to match 5.0.0~alpha0 (bump magic number to match 5.0.0~alpha0 #1288)" is last good commit
• 78ba544 "Runtime: update io.js, fix md5.js" is first bad commit, resulting in the pos_in error.

Once I patch 78ba544 with this patch, I now get:

Uncaught exception TypeError: e.set is not a function.

when calling Sys_js.read_file "ltac_plugin.cma.js".

So maybe the patch is not compatible with 4.1.0-dev, even if it applies. Going to master, I get a different error:

"Uncaught exception TypeError: h is undefined."

This is when calling

    Js.Unsafe.((eval_string js_code : < .. > Js.t -> unit) global);

for the compiled code of ltac_plugin.cma (using --wrap-with-fun)

But likely, it seems like the above code is not the right way to load plugins in JSOO 5.8.

I dunno how to proceed further, I guess I could try to do a variation of this patch on top of 78ba544 so that things work?