Guard for empty singleton in dictionary builder

Author: Azoy

stdlib/public/core/DictionaryBuilder.swift

@@ -102,6 +102,13 @@ extension _NativeDictionary {
     ) -> Int
   ) {
     self.init(capacity: capacity)
+
+    // If the capacity is 0, then our storage is the empty singleton. Those are
+    // read only, so we shouldn't attempt to write to them.
+    if capacity == 0 {
+      return
+    }
+
     let initializedCount = initializer(
       UnsafeMutableBufferPointer(start: _keys, count: capacity),
       UnsafeMutableBufferPointer(start: _values, count: capacity))