Maximum stack size may not fit in unsigned short

Therefore, fields ftransfer/ntransfer in lua_Debug must have type
'int'. (Maximum stack size must fit in an 'int'.) Also, this commit
adds check that maximum stack size respects size_t for size in bytes.

lua.h

@@ -503,8 +503,8 @@ struct lua_Debug {
   unsigned char nparams;/* (u) number of parameters */
   char isvararg;        /* (u) */
   char istailcall;	/* (t) */
-  unsigned short ftransfer;   /* (r) index of first value transferred */
-  unsigned short ntransfer;   /* (r) number of transferred values */
+  int ftransfer;   /* (r) index of first value transferred */
+  int ntransfer;   /* (r) number of transferred values */
   char short_src[LUA_IDSIZE]; /* (S) */
   /* private part */
   struct CallInfo *i_ci;  /* active function */