Don’t free initial src from @INC sub too early

As noted in <20130919225357.GA18474@mars.tony.develop-help.com>, com-
mit 839a0e5 introduces uninitialized warnings (and sometimes other
warnings) in inccode.t.

What actually is happening is that \PVBM is no longer being inlined
and returning the same reference each time, but is returning a new
mortal reference.  Commit 839a0e5 uncovers an existing bug.

A sub in @INC can return a reference to a scalar containing initial
source code.  That returned value would be freed prematurely if it was
not referenced elsewhere.

We have to increment its reference count before leaving the scope, and
then mortalise it afterwards.
perldelta for the prev. commit