FIx a bug for a non-simdlib code of ResidualQuantizer (#3868)

Summary:
This causes an access violation error.

The reason why this was not caught in unit tests for AVX/NEON is that this code branch is unlikely to be used.

The reason why this was not caught in unit tests for a plain non-SIMD binary is unclear.

More ResidualQuantizer patches to follow.

Pull Request resolved: #3868

Reviewed By: mengdilin

Differential Revision: D62882531

Pulled By: mnorris11

fbshipit-source-id: fc50c7409d6064605f783c342b0d313145ffe948

faiss/impl/residual_quantizer_encode_steps.cpp

@@ -95,7 +95,7 @@ void accum_and_store_tab(
         for (size_t ij = 1; ij < M; ij++) {
             reg += cbs[ij][kk];
         }
-        output[b * K + kk] = reg;
+        output[kk] = reg;
     }
 }
 
@@ -152,7 +152,7 @@ void accum_and_add_tab(
         for (size_t ij = 1; ij < M; ij++) {
             reg += cbs[ij][kk];
         }
-        output[b * K + kk] += reg;
+        output[kk] += reg;
     }
 }