[BUG] It is possible to modify v: dictionary with map() and filter() functions

[ZyX]

The following scripts lead to vim crash (ABRT for the first script and SEGV for 
the second):

    # First:
    vim -u NONE -c 'call filter(v:, 0)'
    # Second:
    vim -u NONE -c 'set nocompatible' \
                -c 'call map(v:, "{}")' \
                -c 'nnoremap j :<C-u>echo v:count<CR>' \
                -c 'call feedkeys("1j")'

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