xz: Avoid arithmetic on a null pointer

It's undefined behavior. The result wasn't ever used as it occurred
in the last iteration of a loop.

Clang 17 with -fsanitize=address,undefined:

    $ src/xz/xz --block-list=123
    src/xz/args.c:164:12: runtime error: applying non-zero offset 1
        to null pointer

Fixes: 88ccf47205
Co-authored-by: Sam James <sam@gentoo.org>

src/xz/args.c

@@ -155,6 +155,12 @@ parse_block_list(const char *str_const)
 			}
 		}
 
+		// Be standards compliant: p + 1 is undefined behavior
+		// if p == NULL. That occurs on the last iteration of
+		// the loop when we won't care about the value of str
+		// anymore anyway. That is, this is done conditionally
+		// solely for standard conformance reasons.
+		if (p != NULL)
 			str = p + 1;
 	}