pythongh-124621: Emscripten: Fix regression in use-after-close error handling

Introduced in pythonGH-136822. If the fd is already closed,
`SYSCALLS.getStreamFromFD()` throws an error. We need to catch it and
discard it. The `__wasi_fd_read_orig()` fallback will use it to set errno.

Author: hoodmane

Lib/test/test_fileio.py

@@ -348,8 +348,13 @@ def ReopenForRead(self):
 
     @ClosedFDRaises
     def testErrnoOnClosedRead(self, f):
+        import posix
+        posix._emscripten_debugger()
+        # print("testErrnoOnClosedRead")
         f = self.ReopenForRead()
         f.read(1)
+        print("testErrnoOnClosedRead")
+
 
     @ClosedFDRaises
     def testErrnoOnClosedReadall(self, f):

Python/emscripten_syscalls.c

@@ -148,10 +148,16 @@ EM_JS_MACROS(__externref_t, __maybe_fd_read_async, (
     size_t iovcnt,
     __wasi_size_t *nread
 ), {
-    var stream = SYSCALLS.getStreamFromFD(fd);
     if (!WebAssembly.promising) {
         return null;
     }
+    var stream;
+    try {
+        stream = SYSCALLS.getStreamFromFD(fd);
+    } catch (e) {
+        // We'll let fd_read_orig() handle setting errno.
+        return null;
+    }
     if (!stream.stream_ops.readAsync) {
         // Not an async device. Fall back to __wasi_fd_read_orig().
         return null;