Spirit: fixing segfault if start rule of a grammar is not compatible with the grammar itself

[SVN r56213]

libs/spirit

@@ -1 +1 @@
-Subproject commit 4ed2fbbec9e39a4be87ecf95b00810a02b96595e
+Subproject commit 29401380c9997477952c43901afa28e66ef141e7