libsepol: NULL pointer offset fix

On the first loop iteration the variables `r_buf` and `reason_buf_used`
are NULL respective 0.  Please UBSAN by not adding them but instead
directly assign NULL.

    services.c:800:16: runtime error: applying zero offset to null pointer
        #0 0x4d4fce in constraint_expr_eval_reason ./libsepol/src/services.c:800:16
        #1 0x4cf31a in sepol_validate_transition_reason_buffer ./libsepol/src/services.c:1079:8

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libsepol/src/services.c

@@ -797,7 +797,7 @@ static int constraint_expr_eval_reason(context_struct_t *scontext,
 
 		for (x = 0; buffers[x] != NULL; x++) {
 			while (1) {
-				p = *r_buf + reason_buf_used;
+				p = *r_buf ? (*r_buf + reason_buf_used) : NULL;
 				len = snprintf(p, reason_buf_len - reason_buf_used,
 						"%s", buffers[x]);
 				if (len < 0 || len >= reason_buf_len - reason_buf_used) {