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.

From-SVN: r277788