fix heap-buffer-overflow (asan) in EliasFanoCoding

Test Plan:
fbconfig folly/experimental/test:elias_fano_test && fbmake runtests_opt

Reviewed By: tudorb@fb.com

FB internal diff: D962971

folly/experimental/EliasFanoCoding.h

@@ -132,13 +132,15 @@ struct EliasFanoCompressedList {
 
     // *** Lower bits.
     const size_t lowerSize = (numLowerBits * size + 7) / 8;
-    unsigned char* const lower =
-      static_cast<unsigned char*>(calloc(lowerSize + 7, 1));
+    unsigned char* lower = nullptr;
+    if (lowerSize > 0) {  // numLowerBits != 0
+      lower = static_cast<unsigned char*>(calloc(lowerSize + 7, 1));
       const ValueType lowerMask = (ValueType(1) << numLowerBits) - 1;
       for (size_t i = 0; i < size; ++i) {
         const ValueType lowerBits = list[i] & lowerMask;
         writeBits56(lower, i * numLowerBits, numLowerBits, lowerBits);
       }
+    }
 
     // *** Upper bits.
     // Upper bits are stored using unary delta encoding.