Fix more zero length memset warnings.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@128543 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/ADT/DenseMap.h

@@ -422,7 +422,8 @@ private:
     }
 
 #ifndef NDEBUG
-    memset(OldBuckets, 0x5a, sizeof(BucketT)*OldNumBuckets);
+    if (OldNumBuckets)
+      memset(OldBuckets, 0x5a, sizeof(BucketT)*OldNumBuckets);
 #endif
     // Free the old table.
     operator delete(OldBuckets);