[perl #121362] overload optimisation added a SEGV

My recent commit 3d147ac29d12abdb to "speed up (non)overloaded derefs"
introduced a potential SEGV. In Perl_Gv_AMupdate(), the 'aux' variable is
set to HvAUX(hv). My patch used the value of the variable later on in the
function, but it turns out that by then, S_hsplit() may have been called,
and thus HvARRAY (and HvAUX()) may have been reallocated.

Issue first spotted by Andreas' awesome BBC service, and diagnosed by
Nicholas Clark.