[perl #113060] Save cop_stashlen threaded even with shared cop pv

[perl #113060] Save cop_stashlen threaded even with shared cop pv

Perl_sv_compile_2op_is_broken() does at line 3354 a LEAVE_with_name("eval"),
a SSPOPSTR via SAVEt_SHARED_PVREF for the localized cop_stashpv, but
not for the cop_stashlen.
The cop in question is PL_compiling, which was "AutoSplit" before with
len=9 and restores it back to "main" but keeps len 9. Thus leading to a
heap-overflow in gv_stashpvn.