Always free allocaed memory in mmunmap.

Without this change we were returning early from `__syscall_munmap`
if `_munmap_js` failed.  This could happen if `fd` was no longer a valid
open file descriptor.  However, even in this case we still want to go
ahead and free any allocated memory.

Fixes: emscripten-core#21360

src/library_fs.js

@@ -1276,7 +1276,6 @@ FS.staticInit();` +
       }
       return stream.stream_ops.msync(stream, buffer, offset, length, mmapFlags);
     },
-    munmap: (stream) => 0,
     ioctl(stream, cmd, arg) {
       if (!stream.stream_ops.ioctl) {
         throw new FS.ErrnoError({{{ cDefs.ENOTTY }}});

src/library_noderawfs.js

@@ -193,9 +193,6 @@ addToLibrary({
       // should we check if bytesWritten and length are the same?
       return 0;
     },
-    munmap() {
-      return 0;
-    },
     ioctl() {
       throw new FS.ErrnoError({{{ cDefs.ENOTTY }}});
     }

src/library_syscall.js

@@ -159,13 +159,10 @@ var SyscallsLibrary = {
   _munmap_js__i53abi: true,
   _munmap_js: (addr, len, prot, flags, fd, offset) => {
 #if FILESYSTEM && SYSCALLS_REQUIRE_FILESYSTEM
-    if (isNaN(offset)) return {{{ cDefs.EOVERFLOW }}};
     var stream = SYSCALLS.getStreamFromFD(fd);
     if (prot & {{{ cDefs.PROT_WRITE }}}) {
       SYSCALLS.doMsync(addr, stream, len, flags, offset);
     }
-    FS.munmap(stream);
-    // implicitly return 0
 #endif
   },
 

system/lib/libc/emscripten_mmap.c

@@ -73,10 +73,7 @@ int __syscall_munmap(intptr_t addr, size_t length) {
   UNLOCK(lock);
 
   if (!(map->flags & MAP_ANONYMOUS)) {
-    int rtn = _munmap_js(addr, length, map->prot, map->flags, map->fd, map->offset);
-    if (rtn) {
-      return rtn;
-    }
+    _munmap_js(addr, length, map->prot, map->flags, map->fd, map->offset);
   }
 
   // Release the memory.