ec: free plugin memory when it destructs

When sanitizer is enabled, unittest_erasure_code_shec_thread shows,

```
=================================================================
==737674==ERROR: LeakSanitizer: detected memory leaks

Direct leak of 1360 byte(s) in 5 object(s) allocated from:
    #0 0xaaaadddffb08 in operator new(unsigned long) (/root/ceph/build/bin/unittest_erasure_code_shec_thread+0x1bfb08) (BuildId: 187a0067c45bf30f4d0bd2df83a32e0127ef03a1)
    #1 0xffff800bb004 in __erasure_code_init /root/ceph/src/erasure-code/shec/ErasureCodePluginShec.cc:81:36
    #2 0xaaaadde08de0 in thread1(void*) /root/ceph/src/test/erasure-code/TestErasureCodeShec_thread.cc:100:5
    #3 0xffff7f45d5c4 in start_thread nptl/./nptl/pthread_create.c:442:8
    #4 0xffff7f4c5ed8  misc/../sysdeps/unix/sysv/linux/aarch64/clone.S:79

SUMMARY: AddressSanitizer: 1360 byte(s) leaked in 5 allocation(s).
```

When the plugin destructed, memory should be freed without unloading the
dynamic library.

Signed-off-by: Rongqi Sun <[email protected]>

src/erasure-code/ErasureCodePlugin.cc

@@ -15,6 +15,7 @@
  * 
  */
 
+#include <cassert>
 #include <errno.h>
 
 #include "ceph_ver.h"
@@ -39,15 +40,14 @@ ErasureCodePluginRegistry::ErasureCodePluginRegistry() = default;
 
 ErasureCodePluginRegistry::~ErasureCodePluginRegistry()
 {
-  if (disable_dlclose)
-    return;
-
-  for (std::map<std::string,ErasureCodePlugin*>::iterator i = plugins.begin();
-       i != plugins.end();
-       ++i) {
-    void *library = i->second->library;
-    delete i->second;
-    dlclose(library);
+  for (auto& name_plugin : plugins) {
+    auto *plugin = name_plugin.second;
+    assert(plugin);
+    void *library = plugin->library;
+    delete plugin;
+    if (!disable_dlclose) {
+      dlclose(library);
+    }
   }
 }