Fixed a crash when calling ":split".

Maybe I'm wrong and this isn't the right way to fix this crash, so please review this changeset.

     Program received signal SIGSEGV, Segmentation fault.
     0x0000000000437ec8 in updateimage (win=0x6c9220) at draw.c:1117
     1117                            for (i = 0; di->newline[i].startrow < row && di->newline[i + 1].startrow - 1 < row; i++)
     (gdb) print i
     $1 = 1371
     (gdb) print di->newline[1372]
     Cannot access memory at address 0x6e8000