Fix warnings: fold.c: get_foldtext(): Np dereference: FP.

Problem    : Dereference of null pointer @ 1701.
Diagnostic : False positive.
Rationale  : Comparison `last_wp != wp` just after initializing
             `last_wp` to NULL makes the compiler think `wp` can be
             null. Error appears then on codepath assuming comparison is
             false (i.e. `wp` is null).
Resolution : Change order of OR clauses.
             That seems not to give motives for the analyzer to check
             the `wp` null path and removes the warning.
             But potential null dereference is still there, so we add
             the nonnull annotation to `wp` parameter.