Replace log.Fatal with log.Panic

Fixes #92

memmap.go

diff --git a/memmap.go b/memmap.go
index f577ac8..2e259b8 100644
--- a/memmap.go
+++ b/memmap.go
@@ -66,7 +66,7 @@
 	}
 	parent := m.findParent(f)
 	if parent == nil {
-		log.Fatal("parent of ", f.Name(), " is nil")
+		log.Panic("parent of ", f.Name(), " is nil")
 	}
 	mem.RemoveFromMemDir(parent, f)
 	return nil

util.go

diff --git a/util.go b/util.go
index 2fa8402..cec6f8f 100644
--- a/util.go
+++ b/util.go
@@ -46,7 +46,7 @@
 		err = fs.MkdirAll(ospath, 0777) // rwx, rw, r
 		if err != nil {
 			if err != os.ErrExist {
-				log.Fatalln(err)
+				log.Panicln(err)
 			}
 		}
 	}