Prevent crashing when LHSCAN is provided with a cursor missing the field component

CHANGELOG.md

 # Changelog
 All notable changes to this project will be documented in this file.
 
+## Unreleased
+- Prevent crashing when ``LHSCAN`` is provided with a cursor missing the field
+component.
+
 ## 0.3.7 (2019-04-24)
 - Heavy use of lease commands could cause performance degradation and latency
 spikes, due to accumulation of long-lived deletion tombstones on expiration

src/storage/KeyLocators.hh

@@ -193,7 +193,6 @@ public:
   }
 
   void resetField(std::string_view field) {
-    qdb_assert(!field.empty());
     qdb_assert(localityPrefixSize != 0);
 
     keyBuffer.shrink(localityPrefixSize);

test/e2e.cc

@@ -1550,6 +1550,7 @@ TEST_F(Raft_e2e, LocalityHash) {
   replies.emplace_back(tunnel(leaderID)->exec("lhscan", "mykey", "0" ));
   replies.emplace_back(tunnel(leaderID)->exec("lhscan", "mykey", "0", "COUNT", "2" ));
   replies.emplace_back(tunnel(leaderID)->exec("lhscan", "mykey", "next:hint3##f1", "COUNT", "2" ));
+  replies.emplace_back(tunnel(leaderID)->exec("lhscan", "mykey", "next:hint3##", "COUNT", "5"));
 
   ASSERT_REPLY_DESCRIBE(replies[0],
     "1) \"0\"\n"
@@ -1581,6 +1582,13 @@ TEST_F(Raft_e2e, LocalityHash) {
     "   3) \"v3\"\n"
   );
 
+  ASSERT_REPLY_DESCRIBE(replies[3],
+    "1) \"0\"\n"
+    "2) 1) \"hint3\"\n"
+    "   2) \"f1\"\n"
+    "   3) \"v3\"\n"
+  );
+
   // Now test with evil characters, too
   replies.clear();
   replies.emplace_back(tunnel(leaderID)->exec("lhset", "my#key", "f#1", "hint#1", "v1"));