fuzz-bus-match: new fuzzer

This fuzzer is based on test-bus-match. Even the initial corpus is
derived entirely from it.

https://bugzilla.redhat.com/show_bug.cgi?id=1935084 shows an crash
in bus_match_parse(). I checked the coverage stats on oss-fuzz, and
sadly existing fuzzing did not cover this code at all.

test/fuzz/fuzz-bus-match/test.input

@@ -0,0 +1,18 @@
+arg2='wal\'do',sender='foo',type='signal',interface='bar.x',
+arg2='wal\'do2',sender='foo',type='signal',interface='bar.x',
+arg3='test',sender='foo',type='signal',interface='bar.x',
+arg3='test',sender='foo',type='method_call',interface='bar.x',
+
+interface='quux.x'
+interface='bar.x'
+member='waldo',path='/foo/bar'
+path='/foo/bar'
+path_namespace='/foo'
+path_namespace='/foo/quux'
+arg1='two'
+member='waldo',arg2path='/prefix/'
+member=waldo,path='/foo/bar',arg3namespace='prefix'
+arg4has='pi'
+arg4has='pa'
+arg4has='po'
+arg4='pi'