Function luaD_shrinkstack

** If stack size is more than 3 times the current use, reduce that size ** to twice the current use. (So, the final stack size is at most 2/3 the ** previous size, and half of its entries are empty.) ** As a particular case, if stack was handling a stack overflow and now ** it is not, 'max' (limited by LUAI_MAXSTACK) will be smaller than ** stacksize (equal to ERRORSTACKSIZE in this case), and so