[flang][runtime] Don't do partial data transfer on short character read with ADVANCE='NO', PAD='NO'

When a non-advancing formatted READ can't completely fill a CHARACTER
input item with data, and PAD='NO', don't modify the variable at all.
This matters when the error is recoverable.

Fixes https://github.com/llvm/llvm-project/issues/63772

Differential Revision: https://reviews.llvm.org/D155000