fix condition

src/library_syscall.js

diff --git a/src/library_syscall.js b/src/library_syscall.js
index 0601f16..1fe0c70 100644
--- a/src/library_syscall.js
+++ b/src/library_syscall.js
@@ -278,7 +278,7 @@
 #endif
     // TODO: support unmmap'ing parts of allocations
     var info = SYSCALLS.mappings[addr];
-    if (len === 0 || len > info.len || !info) {
+    if (len === 0 || !info || len > info.len) {
       return -{{{ cDefine('EINVAL') }}};
     }
     if (len < info.len) {