cuse(3): Make cuse_vmfree() NULL safe.

MFC after:	1 week
Sponsored by:	NVIDIA Networking

lib/libcuse/cuse.3

@@ -136,6 +136,7 @@ Else this function returns zero.
 .Fn "cuse_vmfree" "void *"
 This function frees memory allocated by
 .Fn cuse_vmalloc .
+This function is NULL safe.
 Note that the
 cuse library will internally not free the memory until the
 .Fn cuse_uninit

lib/libcuse/cuse_lib.c

@@ -269,7 +269,7 @@ cuse_vmfree(void *ptr)
 	int error;
 	int n;
 
-	if (f_cuse < 0)
+	if (f_cuse < 0 || ptr == NULL)
 		return;
 
 	CUSE_LOCK();