Fix testsuite/g++.dg/cpp1y/constexpr-66093.C execution failure...

The constexpr iteration dereferenced an array element past the end of
the array.


for  gcc/testsuite/ChangeLog

	* g++.dg/cpp1y/constexpr-66093.C: Fix bounds issue.

gcc/testsuite/g++.dg/cpp1y/constexpr-66093.C

@@ -19,7 +19,7 @@ private:
 
 constexpr A f() {
     A a{};
-    for (int i = 1; i <= n; i++) {
+    for (int i = 0; i < n; i++) {
         a[i] = i;
     }
     return a;