[flang] Don't crash in error recovery situation

When folding the TRANSFER() intrinsic function, there's a CHECK()
that the initial image was built without error.  However, there is one
condition that elicits a warning -- a derived type SOURCE= with an
allocatable or dynamic component.  Remove this CHECK().

Fixes llvm-test-suite/Fortran/gfortran/regression/transfer_null_1.f90.

Differential Revision: https://reviews.llvm.org/D157336