Fix clang_analyzer failure (#8492)

Summary:
Previously, the following command:
```USE_CLANG=1 TEST_TMPDIR=/dev/shm/rocksdb OPT=-g make -j$(nproc) analyze```
was raising an error/warning the new_mem could potentially be a `nullptr`. This error appeared due to code changes from https://github.com/facebook/rocksdb/issues/8454, including an if-statement containing "`... && new_mem != nullptr && ...`", which made the analyzer believe that past this `if`-statement, a `new_mem==nullptr` was a possible scenario.
This code patch simply introduces `assert`s and removes this condition in the `if`-statement.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8492

Reviewed By: jay-zhuang

Differential Revision: D29571275

Pulled By: bjlemaire

fbshipit-source-id: 75d72246b70ebbbae7dea11ccb5778686d8bcbea

db/db_impl/db_impl_write.cc

@@ -2113,14 +2113,13 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
   }
 
   cfd->mem()->SetNextLogNumber(logfile_number_);
-
+  assert(new_mem != nullptr);
   // By default, it is assumed that the 'old' memtable
   // will be added to the Imm memtable list and will therefore
   // contribute to the Imm memory footprint.
   bool noImmMemoryContribution = false;
   // If MemPurge activated, purge and delete current memtable.
   if (immutable_db_options_.experimental_allow_mempurge &&
-      (new_mem != nullptr) &&
       ((cfd->GetFlushReason() == FlushReason::kOthers) ||
        (cfd->GetFlushReason() == FlushReason::kManualFlush))) {
     Status mempurge_s = MemPurge(cfd, new_mem);
@@ -2135,9 +2134,8 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
       noImmMemoryContribution = true;
     } else {
       // If mempurge failed, go back to regular mem->imm->flush workflow.
-      if (new_mem) {
-        delete new_mem;
-      }
+      assert(new_mem != nullptr);
+      delete new_mem;
       SuperVersion* new_superversion =
           context->superversion_context.new_superversion.release();
       if (new_superversion != nullptr) {
@@ -2145,6 +2143,7 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
       }
       SequenceNumber seq = versions_->LastSequence();
       new_mem = cfd->ConstructNewMemtable(mutable_cf_options, seq);
+      assert(new_mem != nullptr);
       context->superversion_context.NewSuperVersion();
       cfd->imm()->Add(cfd->mem(), &context->memtables_to_free_);
     }