Non null terminated strings in em_asm section

It appeared that `get_section_strings` in `extract_metadata.py` can't handle strings that don't end with \00.

The story is following. We have our own language in which strings are not null terminated and store their length instead. So we have met some problems when upgrading to a newer version of emscripten.

There was a fix written that we use locally. And i would like to ask if it is possible to merge that fix and, if yes, how to do it better.