testsuite: Avoid uninit var in pr60510.f [PR111427]

The uninitialized variable a in pr60510.f can cause
some random failures as exposed in PR111427.  This
patch is to make it initialized accordingly.

	PR testsuite/111427

gcc/testsuite/ChangeLog:

	* gfortran.dg/vect/pr60510.f (test): Init variable a.