Add explicit braces to avoid dangling else in stl_tree.h.  This silences the following warning produced by clang trunk:
Add explicit braces to avoid dangling else in stl_tree.h.  This silences
the following warning produced by clang trunk:

  In file included from /usr/src/sbin/devd/devd.cc:91:
  In file included from /usr/obj/usr/src/tmp/usr/include/c++/4.2/map:64:
  /usr/obj/usr/src/tmp/usr/include/c++/4.2/bits/stl_tree.h:987:2: error: add explicit braces to avoid dangling else [-Werror,-Wdangling-else]
	  else
	  ^

MFC after:	3 days

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