fs/ntfs3: avoid calling run_get_entry() when run == NULL in ntfs_read_run_nb_ra()

When ntfs_read_run_nb_ra() is invoked with run == NULL the code later
assumes run is valid and may call run_get_entry(NULL, ...), and also
uses clen/idx without initializing them. Smatch reported uninitialized
variable warnings and this can lead to undefined behaviour. This patch
fixes it.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202512230646.v5hrYXL0-lkp@intel.com/
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

Author: aalexandrovich

fs/ntfs3/fsntfs.c

@@ -1256,6 +1256,12 @@ int ntfs_read_run_nb_ra(struct ntfs_sb_info *sbi, const struct runs_tree *run,
 
 		} while (len32);
 
+		if (!run) {
+			err = -EINVAL;
+			goto out;
+		}
+
+		/* Get next fragment to read. */
 		vcn_next = vcn + clen;
 		if (!run_get_entry(run, ++idx, &vcn, &lcn, &clen) ||
 		    vcn != vcn_next) {