[Bug #21106] Remove the useless last iteration

When only one element remains, this simply swaps the first identical
element and has no actual effect.

Author: nobu

array.c

@@ -6701,7 +6701,7 @@ rb_ary_shuffle_bang(rb_execution_context_t *ec, VALUE ary, VALUE randgen)
     rb_ary_modify(ary);
     i = len = RARRAY_LEN(ary);
     RARRAY_PTR_USE(ary, ptr, {
-        while (i) {
+        while (i > 1) {
             long j = RAND_UPTO(i);
             VALUE tmp;
             if (len != RARRAY_LEN(ary) || ptr != RARRAY_CONST_PTR(ary)) {