Program crash and file corruption (most data vanished) upon trying to merge erroneous *.fea file and saving *.sfd file.

[hyvyys]

Trying to merge a feature file with the following line:
sub f' [ b f h k l t ]' by [ f_b.liga f_f.liga f_h.liga f_k.liga f_l.liga f_t.liga ];
results in message warning about non-existent glyph names, while I think it should inform of unknown/erroneous substitution type. (At first glance the sub rule seemed okay, but since it doesn't work I now assume it is not, and the program cannot recognize sub type).
Since the features weren't merged, the project should not be modified, however trying to save the project file results in the program crashing and saving only the initial part of the project, consequently erasing the previously saved version.

When reporting a bug/issue:

• Screenshot

• The FontForge version and the operating system you're using
  FF 2016-10-04 Windows 10 Education

• The behavior you expect to see, and the actual behavior
  Warning message has non-existent glyph names error, I'd expect unknown/erroneous substitution type error. The project should not be modified by the unsuccessful feature merging, while it is.

• Steps to reproduce the behavior

• Open the attached sfd file.
• Merge the attached feature file.
• Save the project sfd file. [program crashes]
• Try to reopen the sfd file. [program complains about corrupted file or crashes upon startup]
• Inspect the corrupted file. [only 30 lines or so are present]

• (optional) Possible solution/fix/workaround
  Replace the erroneous line sub f' [ b f h k l t ]' by [ f_b.liga f_f.liga f_h.liga f_k.liga f_l.liga f_t.liga ]; by individual lines for each substitution:
  sub f' b' by f_b.liga; sub f' f' by f_f.liga;
  etc.

Filippa.zip