vcs-svn: make reading of properties binary-safe

svn-fe errors out on revision 59151 of the ASF repository:

 fatal: invalid dump: unexpected end of file

The proximate cause is a property with an embedded NUL character.
Previously such anomalies were ignored but commit c9d1c8ba
(2010-12-28) introduced a check strlen(val) == len to avoid reading
uninitialized data when a property list ends early and unfortunately
this test does not distinguish between "foo" followed by EOF and the
string "foo\0bar\0baz".

Fix it by using buffer_read_binary to read to a strbuf and checking
the actual length read.  Most consumers of properties still use
C-style strings, so in practice an author or log message with embedded
NULs will be truncated, but a least this way svn-fe won't error out
(fixing the regression).

Reported-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

t/t9010-svn-fe.sh

@@ -370,6 +370,33 @@ test_expect_failure 'change file mode but keep old content' '
 	test_cmp hello actual.target
 '
 
+test_expect_success 'NUL in property value' '
+	reinit_git &&
+	echo "commit message" >expect.message &&
+	{
+		properties \
+			unimportant "something with a NUL (Q)" \
+			svn:log "commit message"&&
+		echo PROPS-END
+	} |
+	q_to_nul >props &&
+	{
+		cat <<-\EOF &&
+		SVN-fs-dump-format-version: 3
+
+		Revision-number: 1
+		EOF
+		echo Prop-content-length: $(wc -c <props) &&
+		echo Content-length: $(wc -c <props) &&
+		echo &&
+		cat props
+	} >nulprop.dump &&
+	test-svn-fe nulprop.dump >stream &&
+	git fast-import <stream &&
+	git diff-tree --always -s --format=%s HEAD >actual.message &&
+	test_cmp expect.message actual.message
+'
+
 test_expect_success 'change file mode and reiterate content' '
 	reinit_git &&
 	cat >expect <<-\EOF &&