alloc.c: releasing NULL now works

My initial free list implementation revealed a
bug that I wasn't handling release(NULL) correctly,
resulting in a segfault.

After this commit, all existing tests pass for
liballoc.

1 files changed, 4 insertions, 0 deletions

diff --git a/src/alloc.c b/src/alloc.c
index 830e49f..2f4aa03 100644
--- a/src/alloc.c
+++ b/src/alloc.c
@@ -76,6 +76,10 @@ void *allocate(size_t size) {
 }
 
 void release(void *ptr) {
+  if (!ptr) {
+    return;
+  }
+
   fl_add_node(&free_list, ptr);
   fl_coalesce_nodes(free_list);
 }