Fix UB in fcntl.c (#24140)

Fixes: #24098

Author: sbc100

system/lib/libc/musl/src/fcntl/fcntl.c

@@ -3,22 +3,28 @@
 #include <stdarg.h>
 #include <errno.h>
 #include "syscall.h"
+#include <emscripten/console.h>
 
-#ifdef __EMSCRIPTEN__
-__attribute__((no_sanitize("address")))
-#endif
 int fcntl(int fd, int cmd, ...)
 {
-	unsigned long arg;
+#ifdef __EMSCRIPTEN__
 	// XXX Emscripten: According to the va_arg man page it is undefined behaviour to
 	// read arguments that are not passed.  This can lead to a false positive
 	// in SAFE_HEAP, so avoid it.
+	unsigned long arg = 0;
 	if (cmd != F_GETFL && cmd != F_GETFD && cmd != F_GETOWN) {
 		va_list ap;
 		va_start(ap, cmd);
 		arg = va_arg(ap, unsigned long);
 		va_end(ap);
 	}
+#else
+	unsigned long arg;
+	va_list ap;
+	va_start(ap, cmd);
+	arg = va_arg(ap, unsigned long);
+	va_end(ap);
+#endif
 	if (cmd == F_SETFL) arg |= O_LARGEFILE;
 	if (cmd == F_SETLKW) return syscall_cp(SYS_fcntl, fd, cmd, (void *)arg);
 	if (cmd == F_GETOWN) {