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Fix NUL-termination bug in saving from Javascript.

The JS code that retrieves the save-file data from emcc.c doesn't
receive a separate length value, but instead expects the data to be in
the form of a NUL-terminated string. But emcc.c wasn't NUL-terminating
it, so the save data could come out with random cruft on the end.

--- a/emcc.c

+++ b/emcc.c

@@ -806,11 +806,15 @@

     midend_serialise(me, savefile_write, &ctx);

     size = ctx.pos;

-    /* Second pass, to actually write out the data */

-    ctx.buffer = snewn(size, char);

+    /* Second pass, to actually write out the data. We have to put a

+     * terminating \0 on the end (which we expect never to show up in

+     * the actual serialisation format - it's text, not binary) so

+     * that the Javascript side can easily find out the length. */

+    ctx.buffer = snewn(size+1, char);

     ctx.pos = 0;

     midend_serialise(me, savefile_write, &ctx);

     assert(ctx.pos == size);

+    ctx.buffer[ctx.pos] = '\0';

     return ctx.buffer;

 }