fix segfault in generic functions

See #22

src/parser.cpp

@@ -2861,8 +2861,12 @@ static void clone_subtree_list_ptr(ZigList<AstNode *> **dest_ptr, ZigList<AstNod
 }
 
 static void clone_subtree_field(AstNode **dest, AstNode *src, uint32_t *next_node_index) {
-    *dest = ast_clone_subtree(src, next_node_index);
-    (*dest)->parent_field = dest;
+    if (src) {
+        *dest = ast_clone_subtree(src, next_node_index);
+        (*dest)->parent_field = dest;
+    } else {
+        *dest = nullptr;
+    }
 }
 
 static void clone_subtree_tld(TopLevelDecl *dest, TopLevelDecl *src, uint32_t *next_node_index) {

test/self_hosted.zig

@@ -542,6 +542,18 @@ fn generic_function_equality() {
 }
 
 
+#attribute("test")
+fn generic_malloc_free() {
+    const a = %%mem_alloc(u8)(10);
+    mem_free(u8)(a);
+}
+const some_mem : [100]u8 = undefined;
+fn mem_alloc(T: type)(n: isize) -> %[]T {
+    return (&T)(&some_mem[0])[0...n];
+}
+fn mem_free(T: type)(mem: []T) { }
+
+
 fn assert(b: bool) {
     if (!b) unreachable{}
 }