When a mesh is pinned in the properties panel, Blender crashes when you
click the "Add Custom Split Normals Data".
The code calls `ob = ED_object_context(C)` which returns NULL when the
mesh is pinned in the properties panel, causing a segfault when trying
to get the mesh via `ob->data`.
A new function `ED_mesh_context(C)` avoids this by first checking
whether a mesh was pinned in the context. If not, it checks the pinned
object's data. If that's not there, or it's not a mesh, it returns the
active object's mesh. Finally it returns NULL if there is no active
object, or if the active object is not a mesh object.
Reviewed By: brecht, mont29
Differential Revision: https://developer.blender.org/D5223
e6e69a28ab