sd-bus: stop using the result of an assignment as an operand of &&

This makes OBJECT_PATH_FOREACH_PREFIX consistent with PATH_FOREACH_PREFIX
and also fixes 7 alerts reported by LGTM at
https://lgtm.com/projects/g/systemd/systemd/snapshot/ac0a08700344a5690803df8dd80e8bb5013184a5/files/src/libsystemd/sd-bus/bus-objects.c?sort=name&dir=ASC&mode=heatmap&showExcluded=true#V1383

src/libsystemd/sd-bus/bus-internal.h

@@ -384,7 +384,7 @@ void bus_close_io_fds(sd_bus *b);
 
 #define OBJECT_PATH_FOREACH_PREFIX(prefix, path)                        \
         for (char *_slash = ({ strcpy((prefix), (path)); streq((prefix), "/") ? NULL : strrchr((prefix), '/'); }) ; \
-             _slash && !(_slash[(_slash) == (prefix)] = 0);             \
+             _slash && ((_slash[(_slash) == (prefix)] = 0), true);       \
              _slash = streq((prefix), "/") ? NULL : strrchr((prefix), '/'))
 
 /* If we are invoking callbacks of a bus object, ensure unreffing the