Avoid crash on moving cursor near the top of file

Crash occurs when cursor is moved near the top of file, which has short first line, and it happens on both of Xcode 5 and Xcode 6 beta. This patch is a quick fix, because I could not understand how to fix `xvim_sb_init`. The cause of this crash is `sb->s_index` sometimes becomes negative value in `xvim_sb_init`, then `xvim_sb_find_backward` makes infinite loop.

```objectivec
NS_INLINE void xvim_sb_init(xvim_string_buffer_t *sb, NSString *s, NSUInteger index, NSRange forRange)
{
    sb->s = s;
    sb->s_min = forRange.location;
    sb->s_max = sb->s_min + forRange.length;

    NSCAssert(index >= sb->s_min && index <= sb->s_max, @"bad caller");

    if (forRange.length < _xvim_sb_size() || index - _xvim_sb_size() / 2 < sb->s_min) {
        sb->s_index = sb->s_min;
    } else if (index + _xvim_sb_size() >= sb->s_max) {
        sb->s_index = sb->s_max - _xvim_sb_size() + 1;
    } else {
        // crash if: -2 = 14 - 16
        sb->s_index = index - _xvim_sb_size() / 2;
    }
    sb->b_index = index - sb->s_index;
    _xvim_sb_load(sb);
}
```

```
Process:               Xcode [2357]
Path:                  /Applications/Xcode6-Beta.app/Contents/MacOS/Xcode
Identifier:            com.apple.dt.Xcode
Version:               6.0 (6194.21)
Build Info:            IDEFrameworks_KLONDIKE-6194021000000000~12
Code Type:             X86-64 (Native)
Parent Process:        ??? [1]
Responsible:           Xcode [2357]
User ID:               501

PlugIn Path:             /Users/USER/Library/Application Support/Developer/*/XVim
PlugIn Identifier:       net.JugglerShu.XVim
PlugIn Version:          1.01 (1)

Date/Time:             2014-06-07 17:44:47.169 +0900
OS Version:            Mac OS X 10.10 (14A238x)
Report Version:        11
Anonymous UUID:        BE6C6651-A72A-54DA-F25E-CEA4C7FE109C


Time Awake Since Boot: 2400 seconds

Crashed Thread:        0  Dispatch queue: com.apple.main-thread

Exception Type:        EXC_BAD_ACCESS (SIGSEGV)
Exception Codes:       KERN_INVALID_ADDRESS at 0x00007ff1f93fffef

VM Regions Near 0x7ff1f93fffef:
    MALLOC_TINY            00007ff1ed000000-00007ff1ed100000 [ 1024K] rw-/rwx SM=PRV  
--> 
    MALLOC_TINY            00007ff1f9400000-00007ff1f9500000 [ 1024K] rw-/rwx SM=PRV  

Application Specific Information:
ProductBuildVersion: 6A215l

Thread 0 Crashed:: Dispatch queue: com.apple.main-thread
0   com.apple.CoreFoundation      	0x00007fff8d956440 __CFStrConvertBytesToUnicode + 16
1   com.apple.CoreFoundation      	0x00007fff8d97752c _CFStringCheckAndGetCharacters + 236
2   com.apple.CoreFoundation      	0x00007fff8d9773e2 -[__NSCFString getCharacters:range:] + 34
3   net.JugglerShu.XVim           	0x0000000118d18c7e -[DVTTextStorage(XVimTextStoring) xvim_indexRangeForLineNumber:newLineLength:] + 761 (DVTTextStorage+XVimTextStoring.m:44)
4   net.JugglerShu.XVim           	0x0000000118d19e5e -[NSTextStorage(VimOperation) xvim_indexOfLineNumber:] + 27 (NSTextStorage+VimOperation.m:132)
5   net.JugglerShu.XVim           	0x0000000118d1a38e -[NSTextStorage(VimOperation) xvim_indexOfLineNumber:column:] + 39 (NSTextStorage+VimOperation.m:204)
6   net.JugglerShu.XVim           	0x0000000118d26067 -[NSTextView(VimOperationPrivate) xvim_getMotionRange:Motion:] + 712 (NSTextView+VimOperation.m:2174)
7   net.JugglerShu.XVim           	0x0000000118d1ffd9 -[NSTextView(VimOperation) xvim_move:] + 59 (NSTextView+VimOperation.m:626)
8   net.JugglerShu.XVim           	0x0000000118cee5c7 -[XVimNormalEvaluator motionFixed:] + 43 (XVimNormalEvaluator.m:516)
9   net.JugglerShu.XVim           	0x0000000118cf95c4 -[XVimNumericEvaluator eval:] + 330 (XVimNumericEvaluator.m:47)
10  net.JugglerShu.XVim           	0x0000000118d09e3a -[XVimWindow handleKeyStroke:onStack:] + 232 (XVimWindow.m:237)
11  net.JugglerShu.XVim           	0x0000000118d09940 -[XVimWindow handleOneXVimString:] + 333 (XVimWindow.m:183)
12  net.JugglerShu.XVim           	0x0000000118d09b48 -[XVimWindow handleXVimString:] + 219 (XVimWindow.m:202)
13  net.JugglerShu.XVim           	0x0000000118d097e7 -[XVimWindow handleKeyEvent:] + 174 (XVimWindow.m:170)
```