[perl #88138] ' is not equivalent to :: before a null

[perl #88138] ' is not equivalent to :: before a null

' is usually equivalent to :: in a symbol name. At the *end* of a
symbol name (as in $'), it’s not equivalent. (That’s a feature.)
Perl_gv_fetchpvn_flags was checking for the end by saying
if(next character), which returns false for a null. It should be
checking the length, which it was already doing for ::, and which
this commit makes it do.