Fixes to pull request #3269 to pass test_mmap, test_mmap_file and test_freetype.

Author: juj

src/library.js

@@ -2572,6 +2572,7 @@ LibraryManager.library = {
     return ptr;
   },
 
+  munmap__deps: ['msync'],
   munmap: function(start, num) {
     if (!_mmap.mappings) _mmap.mappings = {};
     // TODO: support unmmap'ing parts of allocations

src/library_fs.js

@@ -1155,7 +1155,7 @@ mergeInto(LibraryManager.library, {
       return stream.stream_ops.mmap(stream, buffer, offset, length, position, prot, flags);
     },
     msync: function(stream, buffer, offset, length, mmapFlags) {
-      if (!stream.stream_ops.msync) {
+      if (!stream || !stream.stream_ops.msync) {
         return 0;
       }
       return stream.stream_ops.msync(stream, buffer, offset, length, mmapFlags);