BUG: Fix crash in error message formatting introduced by gh-11230

Fixes gh-11221, and now produces:

```python
>>> a = np.ones((3, 3, 4, 5, 6))
>>> b = np.ones((3, 4, 5))
>>> np.einsum('aabcb,abc', a, b)
ValueError: dimensions in operand 0 for collapsing index 'b' don't match (4 != 6)
```

1 files changed, 2 insertions, 1 deletions

diff --git a/numpy/core/src/multiarray/einsum.c.src b/numpy/core/src/multiarray/einsum.c.src
index eb2b33870..e7bbc3d0b 100644
--- a/numpy/core/src/multiarray/einsum.c.src
+++ b/numpy/core/src/multiarray/einsum.c.src
@@ -2156,10 +2156,11 @@ get_combined_dims_view(PyArrayObject *op, int iop, char *labels)
 
             icombinemap[idim] = -1;
             if (new_dims[i] != dim) {
+                char orig_label = labels[idim + label];
                 PyErr_Format(PyExc_ValueError,
                              "dimensions in operand %d for collapsing "
                              "index '%c' don't match (%d != %d)",
-                             iop, label, (int)new_dims[i], (int)dim);
+                             iop, orig_label, (int)new_dims[i], (int)dim);
                 return NULL;
             }
             new_strides[i] += stride;