[stdlib] fix an indexing inconsistency

This iteration ranged from `buffer.startIndex` to `buffer.count`, rather than to `buffer.endIndex`.
Using `buffer.indices` is a better solution in any case.

Author: glessard

stdlib/public/core/Sequence.swift

@@ -1150,7 +1150,7 @@ extension Sequence {
   ) -> (Iterator, UnsafeMutableBufferPointer<Element>.Index) {
     var it = self.makeIterator()
     guard var ptr = buffer.baseAddress else { return (it, buffer.startIndex) }
-    for idx in buffer.startIndex..<buffer.count {
+    for idx in buffer.indices {
       guard let x = it.next() else {
         return (it, idx)
       }