winmm: Don't convert return string in mciSendStringA on error.

dlls/winmm/mci.c

@@ -1506,7 +1506,7 @@ DWORD WINAPI mciSendStringA(LPCSTR lpstrCommand, LPSTR lpstrRet,
         }
     }
     ret = mciSendStringW(lpwstrCommand, lpwstrRet, uRetLen, hwndCallback);
-    if (lpwstrRet)
+    if (!ret && lpwstrRet)
         WideCharToMultiByte( CP_ACP, 0, lpwstrRet, -1, lpstrRet, uRetLen, NULL, NULL );
     HeapFree(GetProcessHeap(), 0, lpwstrCommand);
     HeapFree(GetProcessHeap(), 0, lpwstrRet);

dlls/winmm/tests/mci.c

@@ -53,7 +53,7 @@ START_TEST(mci)
     memset(buf, 0, sizeof(buf));
     err = mciSendString(command_close_all, buf, sizeof(buf), hwnd);
     todo_wine ok(!err,"mciSendString(%s, buf, sizeof(buf) , NULL) returned error: %d\n", command_close_all, err);
-    todo_wine ok(buf[0] == 0, "mciSendString(%s, buf, sizeof(buf) , NULL) changed output buffer: %s\n", command_close_all, buf);
+    ok(buf[0] == 0, "mciSendString(%s, buf, sizeof(buf) , NULL) changed output buffer: %s\n", command_close_all, buf);
 
     memset(buf, 0, sizeof(buf));
     err = mciSendString(command_sysinfo, buf, sizeof(buf), NULL);