[mlir][Linalg] Fix out of bounds access while handling multiple results.

Previous change (a7bfdc23) added
support for fusion of `linalg.generic` op with `tensor.expand_shape`
op when the former had multiple results. Fix a bug related to this
that resulted in a segfault.

Reviewed By: hanchung

Differential Revision: https://reviews.llvm.org/D132631