various: do not propagate exception through dtor

Coverity 189689, 189690, 189692, 189695

contrib/epee/include/mlocker.h

@@ -73,7 +73,7 @@ namespace epee
     mlocked(const T &&t): T(t) { mlocker::lock(this, sizeof(T)); }
     mlocked(const mlocked<T> &&mt): T(mt) { mlocker::lock(this, sizeof(T)); }
     mlocked<T> &operator=(const mlocked<T> &mt) { T::operator=(mt); return *this; }
-    ~mlocked() { mlocker::unlock(this, sizeof(T)); }
+    ~mlocked() { try { mlocker::unlock(this, sizeof(T)); } catch (...) { /* do not propagate */ } }
   };
 
   template<typename T>

contrib/epee/src/mlocker.cpp

@@ -108,7 +108,8 @@ namespace epee
 
   mlocker::~mlocker()
   {
-    unlock(ptr, len);
+    try { unlock(ptr, len); }
+    catch (...) { /* ignore and do not propagate through the dtor */ }
   }
 
   void mlocker::lock(void *ptr, size_t len)

src/common/http_connection.h

@@ -55,7 +55,8 @@ public:
   {
     if (m_ok)
     {
-      mp_http_client->disconnect();
+      try { mp_http_client->disconnect(); }
+      catch (...) { /* do not propagate through dtor */ }
     }
   }
 

src/wallet/wallet2.cpp

@@ -815,7 +815,12 @@ wallet_keys_unlocker::~wallet_keys_unlocker()
 {
   if (!locked)
     return;
-  w.encrypt_keys(key);
+  try { w.encrypt_keys(key); }
+  catch (...)
+  {
+    MERROR("Failed to re-encrypt wallet keys");
+    // do not propagate through dtor, we'd crash
+  }
 }
 
 wallet2::wallet2(network_type nettype, uint64_t kdf_rounds, bool unattended):