fixed undefined behavior

when copying from null pointer
as pointed out here:
TryGhost#1832

Author: gms1

src/statement.h

@@ -57,7 +57,9 @@ namespace Values {
                 Field(_name, SQLITE_BLOB), length(len) {
             value = new char[len];
             assert(value != nullptr);
-            memcpy(value, val, len);
+            if (len > 0) {
+                memcpy(value, val, len);
+            }
         }
         inline virtual ~Blob() override {
             delete[] value;

test/blob.test.js

@@ -51,4 +51,19 @@ describe('blob', function() {
             done();
         });
     });
+
+    it('should be able to select empty blobs', function (done) {
+        const empty = new sqlite3.Database(':memory:');
+        empty.serialize(function () {
+            empty.run("CREATE TABLE files (id INTEGER PRIMARY KEY, data BLOB)");
+            empty.run("INSERT INTO files (data) VALUES (X'')");
+        });
+        empty.get("SELECT data FROM files LIMIT 1", (err, row) => {
+            if (err) throw err;
+            assert.ok(Buffer.isBuffer(row.data));
+            assert.equal(row.data.length, 0);
+            empty.close(done);
+        });
+    });
+
 });