[bootstrap-O3] add a default initializer to avoid a warning at -O3

[bootstrap-O3] add a default initializer to avoid a warning at -O3

Building with the bootstrap-O3 configuration option fails to compile
input.c due to an AFAICT false-positive warning about an uninitialized
use of a variable.

This patch adds a default initializer to silence it.

for  gcc/ChangeLog

	* input.c (assert_char_at_range): Default-initialize
	actual_range.

From-SVN: r244091