Crash on visual deletion of first or last lines in a file

[jrfondren]

replication (last line)

1. start moe without a file argument
2. o
4. v
5. d

This leads (by step 3) to a buffer consisting of

1
2 <CURSOR>

And crashes with

moe.nim(51)              moe
moe.nim(42)              main
visualmode.nim(179)      visualMode
visualmode.nim(118)      visualCommand
visualmode.nim(76)       deleteBuffer
visualmode.nim(62)       yankBuffer
system.nim(2918)         sysFatal
Error: unhandled exception: index out of bounds [IndexError]

replication (first line)

1. start moe without a file argument
2. o
3. 1
5. k
6. v
7. d

This leads (by step 5) to a buffer consisting of

1 <CURSOR>
2 1

And crashes with

moe.nim(88)              moe
moe.nim(75)              main
visualmode.nim(184)      visualMode
visualmode.nim(120)      visualCommand
visualmode.nim(76)       deleteBuffer
visualmode.nim(62)       yankBuffer
system.nim(2918)         sysFatal
Error: unhandled exception: index out of bounds [IndexError]

other info

The failure is seen with both 'master' and 'develop' builds.