Fix UB in test_unistd_write_broken_link (emscripten-core#23125)

The test was failing under ASan because no null byte was being
read from the file, so the strlen inside printf was running
off the buffer. It just happened to work on non-ASan because
wasm memory is initialized to zero.

Author: dschuff

test/unistd/test_unistd_write_broken_link.c

@@ -1,3 +1,4 @@
+#include <assert.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <stdio.h>
@@ -16,15 +17,19 @@ int main() {
     int target_fd = open("link_target", O_RDONLY);
     printf("target_fd: %d, errno: %d %s\n", target_fd, errno, strerror(errno));
     char buf[10];
-    read(target_fd, buf, 10);
+    memset(buf, 0, 10);
+    size_t r = read(target_fd, buf, 10);
+    assert(r == 3);
     printf("buf: '%s'\n", buf);
     close(target_fd);
   }
   {
     int target_fd = open("link_source", O_RDONLY);
     printf("target_fd: %d, errno: %d %s\n", target_fd, errno, strerror(errno));
     char buf[10];
-    read(target_fd, buf, 10);
+    memset(buf, 0, 10);
+    size_t r = read(target_fd, buf, 10);
+    assert(r == 3);
     printf("buf: '%s'\n", buf);
     close(target_fd);
   }