[c++] [testsuite] adjust contracts9.C for negative addresses

The test expected the address of a literal string, converted to long
long, to yield a positive value.  That expectation doesn't necessarily
hold, and the test fails where it doesn't.

Adjust the test to use a pointer that will compare as expected.

for  gcc/testsuite/ChangeLog

	* g++.dg/contracts/contracts9.C: Don't assume string literals
	have non-negative addresses.