fix: Handle NULL pointer dereferencing in libx52

libx52_exit dereferences the device pointer to deinitialize libusb.
However, a user could pass NULL to this function, resulting in a null
pointer dereference.

libx52/Makefile.am

@@ -12,7 +12,7 @@ lib_LTLIBRARIES += libx52.la
 # See: https://www.gnu.org/software/libtool/manual/html_node/Updating-version-info.html
 libx52_v_CUR=6
 libx52_v_AGE=4
-libx52_v_REV=1
+libx52_v_REV=2
 libx52_la_SOURCES = \
 	libx52/x52_control.c \
 	libx52/x52_core.c \

libx52/x52_core.c

@@ -229,6 +229,10 @@ int libx52_init(libx52_device **dev)
 
 void libx52_exit(libx52_device *dev)
 {
+    if (!dev) {
+        return;
+    }
+
     libx52_disconnect(dev);
     libusb_exit(dev->ctx);