astfd.c: Avoid calling fclose with NULL argument.

Don't pass through a NULL argument to fclose, which is undefined
behavior, and instead return -1 and set errno appropriately. This
also avoids a compiler warning with glibc 2.38 and newer, as glibc
commit 71d9e0fe766a3c22a730995b9d024960970670af
added the nonnull attribute to this argument.

Resolves: #900

main/astfd.c

@@ -280,7 +280,8 @@ int __ast_fdleak_fclose(FILE *ptr)
 {
 	int fd, res;
 	if (!ptr) {
-		return fclose(ptr);
+		errno = EINVAL;
+		return -1;
 	}
 
 	fd = fileno(ptr);