fix the error searching for the empty slot

Author: MaJerle

lwmem/src/lwmem/lwmem.c

@@ -239,7 +239,7 @@ prv_insert_free_block(lwmem_t* const lwobj, lwmem_block_t* nblk) {
      * Try to find position to put new block in-between
      * Search until all free block addresses are lower than entry block
      */
-    for (prev = &(lwobj->start_block); prev != NULL && prev->next < nblk; prev = prev->next) {}
+    for (prev = &(lwobj->start_block); prev != NULL && prev->next != NULL && prev->next < nblk; prev = prev->next) {}
 
     /* This is hard error with wrong memory usage */
     if (prev == NULL) {