LibC: Don't format strings when asserting with an unstable heap

If we hit an assertion while the heap isn't in a stable state, we can't
rely on dynamic memory allocation because the malloc mutex is already
held and the heap is most likely corrupted. Instead, we need to bail
out fast before we make the situation even worse.

Userland/Libraries/LibC/malloc.cpp

@@ -26,8 +26,13 @@ public:
         : m_mutex(mutex)
     {
         lock();
+        __heap_is_stable = false;
+    }
+    ALWAYS_INLINE ~PthreadMutexLocker()
+    {
+        __heap_is_stable = true;
+        unlock();
     }
-    ALWAYS_INLINE ~PthreadMutexLocker() { unlock(); }
     ALWAYS_INLINE void lock() { pthread_mutex_lock(&m_mutex); }
     ALWAYS_INLINE void unlock() { pthread_mutex_unlock(&m_mutex); }
 
@@ -38,6 +43,7 @@ private:
 #define RECYCLE_BIG_ALLOCATIONS
 
 static pthread_mutex_t s_malloc_mutex = PTHREAD_MUTEX_INITIALIZER;
+bool __heap_is_stable = true;
 
 constexpr size_t number_of_hot_chunked_blocks_to_keep_around = 16;
 constexpr size_t number_of_cold_chunked_blocks_to_keep_around = 16;