[committed] pt.c bootstrap warning

[Jim Wilson <wilson at tuliptree dot org>]

This fixes a bug reported by Andrew Pinksi.
    http://gcc.gnu.org/ml/gcc/2008-03/msg01058.html

We are getting a warning during the bootstrap while compiling pt.c.
This is supposed to be an error, but for another bug in the enable
checking code.  Anyways, we have to fix this before we can fix the
enable checking code.

../../gcc/gcc/cp/pt.c: In function âtsubst_copyâ:
../../gcc/gcc/cp/pt.c:9919: warning: âlenâ may be used uninitialized in
this function

This bug was introduced by a patch from Doug Gregor.
    http://gcc.gnu.org/ml/gcc-patches/2008-03/msg01675.html

As best as I can tell, there are 3 possible return values from the
tsubst_pack_expansion call: error_mark_node, a TREE_VEC, or a
PACK_EXPANSION.  Hence the code is OK, gcc just can't tell that len will
always be set on the path where it is used.

So I just checked in a patch to initialize len to zero to avoid the
warning.

This was tested with an ia64-linux C and C++ bootstrap and make check.
There were no regressions, and I also verified that the warning was
gone.

Jim

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