Fix assert failure

Author: multiplemonomials

connectivity/lwipstack/source/LWIPMemoryManager.cpp

@@ -189,5 +189,9 @@ void LWIPMemoryManager::set_total_len(struct pbuf *pbuf)
 
 void mbed_lwip_on_pbuf_pool_free_hook()
 {
-    LWIP::get_instance().get_memory_manager().onPoolSpaceAvailCallback();
+    auto & callback = LWIP::get_instance().get_memory_manager().onPoolSpaceAvailCallback;
+    if(callback)
+    {
+        callback();
+    }
 }

connectivity/nanostack/mbed-mesh-api/source/NanostackMemoryManager.cpp

@@ -158,5 +158,9 @@ NetStackMemoryManager::Lifetime NanostackMemoryManager::get_lifetime(const net_s
 
 void mbed_ns_heap_free_hook()
 {
-    Nanostack::get_instance().get_memory_manager().onPoolSpaceAvailCallback();
+    auto & callback = Nanostack::get_instance().get_memory_manager().onPoolSpaceAvailCallback;
+    if(callback)
+    {
+        callback();
+    }
 }

connectivity/netsocket/tests/emac_test_utils/EmacTestMemoryManager.cpp

@@ -317,7 +317,7 @@ void EmacTestMemoryManager::free(emac_mem_buf_t *buf)
 
     m_mem_mutex.unlock();
 
-    if (pool_buf_freed) {
+    if (pool_buf_freed && onPoolSpaceAvailCallback) {
         onPoolSpaceAvailCallback();
     }
 }
@@ -514,7 +514,7 @@ void EmacTestMemoryManager::set_memory_available(bool memory)
     m_memory_available = memory;
 
     // Poke the EMAC in case it can allocate buffers
-    if (m_memory_available) {
+    if (m_memory_available && onPoolSpaceAvailCallback) {
         onPoolSpaceAvailCallback();
     }
 }