pmcx buffer overflow error if bc parameter has 12 characters

[lkeegan]

Setting 'bc': '______000010' gives:

*** buffer overflow detected ***: terminated

If I remove the last character, 'bc': '______00001', then it runs without errors.

[fangq]

after checking the code, I found the issue you mentioned here also presents in mcxlab (and mcxcl). Because in mcxlab, I have been using cfg->bc as a NULL-terminating string for printing while in nearly all other places, I only read the first 12 bytes for bc flags.

To avoid issues of mixing use as a string as well as a byte array, I decided to expand the bc[] array in cfg to 13 to allow the ending NULL, 68db492

following this change, I also adjusted mcxlab.cpp and pmcx.cpp to handle the size checking and copying accordingly, see my above tagged commit.

Let me know if you see any problem.