Fix conditional expression in musl's dup2. NFC. (#13740)

* Fix conditional expression in musl's dup2. NFC.

The if/else branch of the conditional expression was reversed.

* Use the appropriate error code

0 for success and -1 to indicate an error. Add a test case to catch
this (this previously failed when `SYS_dup2` was `#undef`'d).

Author: kleisauke

system/lib/libc/musl/src/unistd/dup2.c

@@ -13,10 +13,10 @@ int dup2(int old, int new)
 	while ((r=__syscall(SYS_dup2, old, new))==-EBUSY);
 #else
 	if (old==new) {
-#if __EMSCRIPTEN__
-		r = __syscall(SYS_fcntl, old, F_GETFD);
+#ifdef __EMSCRIPTEN__
+		r = __wasi_fd_is_valid(old) ? 0 : -1;
 #else
-		r = __wasi_fd_is_valid(old);
+		r = __syscall(SYS_fcntl, old, F_GETFD);
 #endif
 		if (r >= 0) return old;
 	} else {

tests/unistd/dup.c

@@ -39,5 +39,12 @@ int main() {
   printf("\n");
   errno = 0;
 
+  printf("DUP2 err\n");
+  f = dup2(-2, -2);
+  printf("f: %d\n", f == -1);
+  printf("errno: %d\n", errno);
+  printf("close(f): %d\n", close(f));
+  errno = 0;
+
   return 0;
 }

tests/unistd/dup.out

@@ -13,3 +13,8 @@ f2,f3: 1
 close(f1): 0
 close(f2): 0
 close(f3): -1
+
+DUP2 err
+f: 1
+errno: 8
+close(f): -1