SLP: Initialize variable to fix bootstrap after r277784.

SLP: Initialize variable to fix bootstrap after r277784.

This initializes the rstmt variable with NULL and adds an assert to
check that a value has been given by one of the if cases before use.

This fixes the bootstrap failure due to -Werror.

Committed under the gcc obvious rule.

gcc/ChangeLog:

	* tree-vect-slp.c (vectorize_slp_instance_root_stmt): Initialize rstmt.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@277788 138bc75d-0d04-0410-961f-82ee72b054a4