libstdc++: Fix testsuite with -Wformat

Fix testsuite when compiling with -Wformat. Use nonnull arguments so
that -Wformat does not cause extraneous output to be reported as an
error.

FAIL: tr1/8_c_compatibility/cinttypes/functions.cc (test for excess errors)

libstdc++-v3/ChangeLog:

	* testsuite/tr1/8_c_compatibility/cinttypes/functions.cc: Use
	nonnull arguments to strtoimax() and wcstoimax() functions.

Signed-off-by: Gwenole Beauchesne <gb.devel@gmail.com>