fix harmless invalid read in Perl_re_compile()

fix harmless invalid read in Perl_re_compile()

[perl #2460] described a case where electric fence reported an invalid
read. This could be reproduced under valgrind with blead and -e'/x/',
but only on a non-debugging build.

This was because it was checking for certain pairs of nodes (e.g. BOL + END)
and wasn't allowing for EXACT nodes, which have the string at the next
node position when using a naive NEXTOPER(first). In the non-debugging
build, the nodes aren't initialised to zero, and a 1-char EXACT node isn't
long enough to spill into the type field of the "next node".

Fix this by only using NEXTOPER(first) when we know the first node is
kosher.