Fix unused variable of r in release mode (#6750)

Summary:
In release mode, asserts are not compiled, so `r` is not used, causing compiler warnings.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6750

Test Plan: make check under release mode

Reviewed By: anand1976

Differential Revision: D21220365

Pulled By: cheng-chang

fbshipit-source-id: fd4afa9843d54af68c4da8660ec61549803e1167

env/io_posix.cc

@@ -610,10 +610,9 @@ IOStatus PosixRandomAccessFile::MultiRead(FSReadRequest* reqs,
                                           IODebugContext* dbg) {
   if (use_direct_io()) {
     for (size_t i = 0; i < num_reqs; i++) {
-      const FSReadRequest& r = reqs[i];
+      assert(IsSectorAligned(reqs[i].offset, GetRequiredBufferAlignment()));
-      assert(IsSectorAligned(r.offset, GetRequiredBufferAlignment()));
+      assert(IsSectorAligned(reqs[i].len, GetRequiredBufferAlignment()));
-      assert(IsSectorAligned(r.len, GetRequiredBufferAlignment()));
+      assert(IsSectorAligned(reqs[i].scratch, GetRequiredBufferAlignment()));
-      assert(IsSectorAligned(r.scratch, GetRequiredBufferAlignment()));
     }
   }