libsysprof-capture: ensure trailing bit is zero

This ensures situations that are not perfectly aligned can still be 1-off
checked for a trailing \0 without scanning.

src/libsysprof-capture/sysprof-capture-writer.c

@@ -613,6 +613,8 @@ sysprof_capture_writer_add_map (SysprofCaptureWriter *self,
   _sysprof_strlcpy (ev->filename, filename, len - sizeof *ev);
   ev->filename[len - sizeof *ev - 1] = '\0';
 
+  ((char*)ev)[len-1] = 0;
+
   self->stat.frame_count[SYSPROF_CAPTURE_FRAME_MAP]++;
 
   return true;