tree-optimization: Fix use of uninitialized variable [PR94774]

Array retval is not necessarily initialized by function is_call_safe and
may be used afterwards.  Thus, initialize it explicitly.

gcc/ChangeLog:

2020-04-29  Stefan Schulze Frielinghaus  <stefansf@linux.ibm.com>

	PR tree-optimization/94774
	* gimple-ssa-sprintf.c (try_substitute_return_value): Initialize
	variable retval.