xxd: fix printing of trailing spaces

function                                             old     new   delta
bb_dump_dump                                        1497    1523     +26
xxd_main                                             459     466      +7
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(add/remove: 0/0 grow/shrink: 2/0 up/down: 33/0)               Total: 33 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

testsuite/xxd.tests

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+# Copyright 2020 by Denys Vlasenko <vda.linux@googlemail.com>
+# Licensed under GPLv2, see file LICENSE in this source tree.
+
+. ./testing.sh
+
+# testing "description" "command" "result" "infile" "stdin"
+testing 'xxd -p with one NUL' \
+	'xxd -p' \
+	"\
+00
+" \
+	'' \
+	'\0'
+
+testing 'xxd -p with 30 NULs' \
+	'xxd -p' \
+	"\
+000000000000000000000000000000000000000000000000000000000000
+" \
+	'' \
+	'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0'
+
+testing 'xxd -p with 31 NULs' \
+	'xxd -p' \
+	"\
+000000000000000000000000000000000000000000000000000000000000
+00
+" \
+	'' \
+	'\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0'
+
+exit $FAILCOUNT