Fix potential invalid read in shift_left()

Fix potential invalid read in shift_left()

The second call to shift_left() in graph_generate_symbols() passes as
the index `pos + 1` where `pos` is bounded by `row->size` and will
therefore be an invalid for the last index. The fix is to check the
bounds of `pos + 1` and only conditionally invoke shift_left().

The stacktrace as reported in issue #402:

    #0 0x00000c47bba3a095 in shift_left (row=0xc4a76750c50, prev_row=0xc4a76750c70, pos=64) at src/graph-v2.c:493
    493 if (!graph_column_has_commit(&row->columns[pos]))
    (gdb) bt
    #0 0x00000c47bba3a095 in shift_left (row=0xc4a76750c50, prev_row=0xc4a76750c70, pos=64) at src/graph-v2.c:493
    #1 0x00000c47bba3aad7 in graph_generate_symbols (graph=0xc4a76750c00, canvas=0xc4a5c144450) at src/graph-v2.c:671
    #2 0x00000c47bba3ac86 in graph_render_parents (graph_ref=0xc4a76750c00, canvas=0xc4a5c144450) at src/graph-v2.c:701