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.

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.
+
 2020-04-29  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* calls.h (cxx17_empty_base_field_p): Turn into a function declaration.

gcc/gimple-ssa-sprintf.c

@@ -4120,7 +4120,7 @@ try_substitute_return_value (gimple_stmt_iterator *gsi,
   bool removed = false;
 
   /* The minimum and maximum return value.  */
-  unsigned HOST_WIDE_INT retval[2];
+  unsigned HOST_WIDE_INT retval[2] = {0};
   bool safe = is_call_safe (info, res, true, retval);
 
   if (safe