add validity check AlignedTableTightAlloc clear method (facebookresearch#3997)

Summary:
Pull Request resolved: facebookresearch#3997

Avoid null pointer error by checking that `numel > 0`.
It's equivalent of checking`ptr != null` but keeping numel check for consistency.

Bug reported in: facebookresearch#3994

Reviewed By: mnorris11

Differential Revision: D65073502

fbshipit-source-id: f9aa56cbcd28fc352d45ef5b8727fa857fbe5345

Author: pankajsingh88

faiss/utils/AlignedTable.h

@@ -63,7 +63,9 @@ struct AlignedTableTightAlloc {
     }
 
     void clear() {
-        memset(ptr, 0, nbytes());
+        if (numel > 0) {
+            memset(ptr, 0, nbytes());
+        }
     }
     size_t size() const {
         return numel;