libluna/Heap: Crash the kernel (but not userspace) on invalid frees

This makes them way easier to catch and forces us to get those out of the way.

libluna/src/Heap.cpp

@@ -249,13 +249,21 @@ Result<void> free_impl(void* ptr)
         else
             dbgln("ERROR: Attempt to free memory at %p, which wasn't allocated with malloc", ptr);
 
+#ifdef USE_FREESTANDING
+        fail("Call to free_impl() with an invalid argument (double-free or erroneous deallocation)");
+#else
         return err(EFAULT);
+#endif
     }
 
     if (is_block_free(block))
     {
         dbgln("ERROR: Attempt to free memory at %p, which was already freed", ptr);
+#ifdef USE_FREESTANDING
+        fail("Call to free_impl() with a pointer to freed memory (probably double-free)");
+#else
         return err(EFAULT);
+#endif
     }
     else
         block->status &= ~BLOCK_USED;