maybe fix issue jive-vlbi#24 (SIGSEGV)

Using the coredump at On was able to trace the SIGSEGV *happening in the
garbage collector thread* that the UDT library starts (!):

    6  0x000055e8617dd78e in CSndBuffer::~CSndBuffer (this=0x7f8e7c0021a0, __in_chrg=<optimized out>) at /usr/local/src/jive5ab.git/libudt5ab/buffer.cpp:101
    7  0x000055e8617e208b in CUDT::~CUDT (this=0x55e8652c9780, __in_chrg=<optimized out>) at /usr/local/src/jive5ab.git/libudt5ab/core.cpp:195
    8  0x000055e8617cab58 in CUDTSocket::~CUDTSocket (this=0x55e8652c9650, __in_chrg=<optimized out>) at /usr/local/src/jive5ab.git/libudt5ab/api.cpp:99

so it's likely that ~CSndBuffer() is being executed from different
threads and the d'tor has no protection at all against that.

It's a probable cause for sigsegv'ing so added a mutex (on non-Win32
systems) to see if that fixes the issue reported

Author: Marjolein Verkouter

libudt5ab/buffer.cpp

@@ -93,28 +93,38 @@ m_iCount(0)
 
 CSndBuffer::~CSndBuffer()
 {
-   Block* pb = m_pBlock->m_pNext;
-   while (pb != m_pBlock)
-   {
-      Block* temp = pb;
-      pb = pb->m_pNext;
-      delete temp;
-   }
-   delete m_pBlock;
-
-   while (m_pBuffer != NULL)
-   {
-      Buffer* temp = m_pBuffer;
-      m_pBuffer = m_pBuffer->m_pNext;
-      delete [] temp->m_pcData;
-      delete temp;
-   }
-
-   #ifndef WIN32
-      pthread_mutex_destroy(&m_BufLock);
-   #else
-      CloseHandle(m_BufLock);
-   #endif
+#ifndef WIN32
+    pthread_mutex_t  dtor_lock = PTHREAD_MUTEX_INITIALIZER;
+    ::pthread_mutex_lock( &dtor_lock );
+#endif
+
+    Block* pb = (m_pBlock != NULL ? m_pBlock->m_pNext : NULL);
+    while (pb != m_pBlock)
+    {
+        Block* temp = pb;
+        pb = pb->m_pNext;
+        delete temp;
+    }
+    delete m_pBlock;
+    m_pBlock = NULL;
+
+    while (m_pBuffer != NULL)
+    {
+        Buffer* temp = m_pBuffer;
+        m_pBuffer = m_pBuffer->m_pNext;
+        delete [] temp->m_pcData;
+        delete temp;
+    }
+
+    #ifndef WIN32
+    pthread_mutex_destroy(&m_BufLock);
+    #else
+    CloseHandle(m_BufLock);
+    #endif
+
+#ifndef WIN32
+    ::pthread_mutex_unlock( &dtor_lock );
+#endif
 }
 
 void CSndBuffer::addBuffer(const char* data, int len, int ttl, bool order)