Fix : Iteration for BMLayerCollection was broken

It was broken in two ways:
- bpy_bmlayercollection_iter passed PY_SSIZE_T_MIN, while
PY_SSIZE_T_MAX was needed.
- bpy_bmlayercollection_subscript_slice() contained an
off-by-one error.

Pull Request: blender/blender#107165

source/blender/python/bmesh/bmesh_py_types_customdata.c

@@ -785,11 +785,11 @@ static PyObject *bpy_bmlayercollection_subscript_slice(BPy_BMLayerCollection *se
 
   BPY_BM_CHECK_OBJ(self);
 
-  if (start >= len) {
-    start = len - 1;
+  if (start > len) {
+    start = len;
   }
-  if (stop >= len) {
-    stop = len - 1;
+  if (stop > len) {
+    stop = len;
   }
 
   tuple = PyTuple_New(stop - start);
@@ -915,7 +915,7 @@ static PyObject *bpy_bmlayercollection_iter(BPy_BMLayerCollection *self)
 
   BPY_BM_CHECK_OBJ(self);
 
-  ret = bpy_bmlayercollection_subscript_slice(self, 0, PY_SSIZE_T_MIN);
+  ret = bpy_bmlayercollection_subscript_slice(self, 0, PY_SSIZE_T_MAX);
 
   if (ret) {
     iter = PyObject_GetIter(ret);