tests: skip g_dbus_message_new_from_blob under asan

Some versions of asan report the following false positive
when strict_string_checks=1 is passed:

=================================================================
==3297==ERROR: AddressSanitizer: SEGV on unknown address 0x000000000000 (pc 0x7f64e4090286 bp 0x7ffe46acd9a0 sp 0x7ffe46acd118 T0)
==3297==The signal is caused by a READ memory access.
==3297==Hint: address points to the zero page.
    #0 0x7f64e4090285 in __strlen_sse2 (/lib64/libc.so.6+0xaa285)
    #1 0x7f64e5a51e46  (/lib64/libasan.so.4+0x41e46)
    #2 0x7f64e4e5e3a0  (/lib64/libglib-2.0.so.0+0x383a0)
    #3 0x7f64e4e5e536 in g_dgettext (/lib64/libglib-2.0.so.0+0x38536)
    #4 0x7f64e48fac5f  (/lib64/libgio-2.0.so.0+0xc1c5f)
    #5 0x7f64e4c03978 in g_type_class_ref (/lib64/libgobject-2.0.so.0+0x30978)
    #6 0x7f64e4be9567 in g_object_new_with_properties (/lib64/libgobject-2.0.so.0+0x16567)
    #7 0x7f64e4be9fd0 in g_object_new (/lib64/libgobject-2.0.so.0+0x16fd0)
    #8 0x7f64e48fd43e in g_dbus_message_new_from_blob (/lib64/libgio-2.0.so.0+0xc443e)
    #9 0x564a6aa0de52 in main ../src/libsystemd/sd-bus/test-bus-marshal.c:228
    #10 0x7f64e4007009 in __libc_start_main (/lib64/libc.so.6+0x21009)
    #11 0x564a6aa0a569 in _start (/home/vagrant/systemd/build/test-bus-marshal+0x5569)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV (/lib64/libc.so.6+0xaa285) in __strlen_sse2
==3297==ABORTING

It's an external library and errors in external libraries are generally not very
useful for looking for internal bugs.

It would be better not to change the code and use standard suppression
techinques decribed at
https://clang.llvm.org/docs/AddressSanitizer.html#suppressing-reports-in-external-libraries,
but, unfortunaley, none of them seems to be able to suppress fatal errors in asan intself.

src/libsystemd/sd-bus/test-bus-marshal.c

@@ -217,6 +217,7 @@ int main(int argc, char *argv[]) {
         free(h);
 
 #if HAVE_GLIB
+#ifndef __SANITIZE_ADDRESS__
         {
                 GDBusMessage *g;
                 char *p;
@@ -232,6 +233,7 @@ int main(int argc, char *argv[]) {
                 g_object_unref(g);
         }
 #endif
+#endif
 
 #if HAVE_DBUS
         {