fix options_test ubsan

Summary:
Having -ve value for max_write_buffer_number does not make sense and cause us to do a left shift on a -ve value number
Closes https://github.com/facebook/rocksdb/pull/1579

Differential Revision: D4240798

Pulled By: IslamAbdelRahman

fbshipit-source-id: bd6267e

util/options_test.cc

@@ -305,8 +305,8 @@ TEST_F(OptionsTest, GetColumnFamilyOptionsFromStringTest) {
 
   // Units (k)
   ASSERT_OK(GetColumnFamilyOptionsFromString(
-      base_cf_opt, "max_write_buffer_number=-15K", &new_cf_opt));
-  ASSERT_EQ(new_cf_opt.max_write_buffer_number, -15 * kilo);
+      base_cf_opt, "max_write_buffer_number=15K", &new_cf_opt));
+  ASSERT_EQ(new_cf_opt.max_write_buffer_number, 15 * kilo);
   // Units (m)
   ASSERT_OK(GetColumnFamilyOptionsFromString(base_cf_opt,
             "max_write_buffer_number=16m;inplace_update_num_locks=17M",