stop calling memcmp with nullptrs

Summary:
it doesn't take nullptr according to its declaration in glibc, and calling it in this way causes our sanitizers (ubsan, clang analyze) to fail.
Closes https://github.com/facebook/rocksdb/pull/2776

Differential Revision: D5683260

Pulled By: ajkr

fbshipit-source-id: 114b137ee188172f96eedc43139255cae7bee80a

include/rocksdb/slice.h

@@ -213,18 +213,17 @@ inline bool operator!=(const Slice& x, const Slice& y) {
   return !(x == y);
 }
 
-// UBSAN complain that we pass nullptr to memcmp that's fine since
-// we always do that for a string of len = 0
-#ifdef ROCKSDB_UBSAN_RUN
-#if defined(__clang__)
-__attribute__((__no_sanitize__("undefined")))
-#elif defined(__GNUC__)
-__attribute__((__no_sanitize_undefined__))
-#endif
-#endif
 inline int Slice::compare(const Slice& b) const {
   const size_t min_len = (size_ < b.size_) ? size_ : b.size_;
-  assert((data_ != nullptr && b.data_ != nullptr) || min_len == 0);
+  if (min_len == 0) {
+    if (size_ > 0) {
+      return 1;
+    } else if (b.size_ > 0) {
+      return -1;
+    }
+    return 0;
+  }
+  assert(data_ != nullptr && b.data_ != nullptr);
   int r = memcmp(data_, b.data_, min_len);
   if (r == 0) {
     if (size_ < b.size_) r = -1;