Fixed joystick vendor detection in Linux automatic gamepad mapping

(cherry picked from commit 071d1e29dd4f80605932f320aa2e05fda383944c)

src/joystick/SDL_joystick.c

@@ -123,10 +123,10 @@ static SDL_atomic_t SDL_next_joystick_instance_id SDL_GUARDED_BY(SDL_joystick_lo
 static int SDL_joystick_player_count SDL_GUARDED_BY(SDL_joystick_lock) = 0;
 static SDL_JoystickID *SDL_joystick_players SDL_GUARDED_BY(SDL_joystick_lock) = NULL;
 static SDL_bool SDL_joystick_allows_background_events = SDL_FALSE;
-static char joystick_magic;
+char SDL_joystick_magic;
 
 #define CHECK_JOYSTICK_MAGIC(joystick, retval)             \
-    if (!joystick || joystick->magic != &joystick_magic) { \
+    if (!joystick || joystick->magic != &SDL_joystick_magic) { \
         SDL_InvalidParamError("joystick");                 \
         SDL_UnlockJoysticks();                             \
         return retval;                                     \
@@ -514,7 +514,7 @@ SDL_Joystick *SDL_JoystickOpen(int device_index)
         SDL_UnlockJoysticks();
         return NULL;
     }
-    joystick->magic = &joystick_magic;
+    joystick->magic = &SDL_joystick_magic;
     joystick->driver = driver;
     joystick->instance_id = instance_id;
     joystick->attached = SDL_TRUE;
@@ -723,7 +723,7 @@ int SDL_JoystickSetVirtualHat(SDL_Joystick *joystick, int hat, Uint8 value)
 SDL_bool SDL_PrivateJoystickValid(SDL_Joystick *joystick)
 {
     SDL_AssertJoysticksLocked();
-    return (joystick && joystick->magic == &joystick_magic);
+    return (joystick && joystick->magic == &SDL_joystick_magic);
 }
 
 SDL_bool SDL_PrivateJoystickGetAutoGamepadMapping(int device_index, SDL_GamepadMapping *out)

src/joystick/SDL_joystick_c.h

@@ -34,6 +34,7 @@ extern "C" {
 #endif
 
 struct _SDL_JoystickDriver;
+extern char SDL_joystick_magic;
 
 /* Initialization and shutdown functions */
 extern int SDL_JoystickInit(void);

src/joystick/linux/SDL_sysjoystick.c

@@ -1708,6 +1708,7 @@ static SDL_bool LINUX_JoystickGetGamepadMapping(int device_index, SDL_GamepadMap
     /* We temporarily open the device to check how it's configured. Make
        a fake SDL_Joystick object to do so. */
     joystick = (SDL_Joystick *)SDL_calloc(sizeof(*joystick), 1);
+    joystick->magic = &SDL_joystick_magic;
     if (joystick == NULL) {
         SDL_OutOfMemory();
         return SDL_FALSE;