Global initialization checks name conflict

Initialization "global a = 10" raises an error if global 'a' is already
defined, that is, it has a non-nil value.

lcode.c

@@ -705,6 +705,22 @@ static void luaK_float (FuncState *fs, int reg, lua_Number f) {
 }
 
 
+/*
+** Get the value of 'var' in a register and generate an opcode to check
+** whether that register is nil. 'k' is the index of the variable name
+** in the list of constants. If its value cannot be encoded in Bx, a 0
+** will use '?' for the name.
+*/
+void luaK_codecheckglobal (FuncState *fs, expdesc *var, int k, int line) {
+  luaK_exp2anyreg(fs, var);
+  luaK_fixline(fs, line);
+  k = (k >= MAXARG_Bx) ? 0 : k + 1;
+  luaK_codeABx(fs, OP_ERRNNIL, var->u.info, k);
+  luaK_fixline(fs, line);
+  freeexp(fs, var);
+}
+
+
 /*
 ** Convert a constant in 'v' into an expression description 'e'
 */