childs.c: fix minor memory leak

this would leak only once on program termination, so it's no big
deal apart from having spurious reachable memory in valgrind logs.

src/child.c

@@ -217,6 +217,7 @@ oom:
                         goto oom;
 		}
         }
+	sblist_free(childs);
 }
 
 /*