Fix segfault in libwebsocket_write()

Fix segfault in libwebsocket_write()

Since 'shift' has unsigned integer type,
the following code may lead to infinite cycle
and segfault:

    while (shift >= 0) {
    	if (shift)
    		buf[0 - pre + n] =
    			((len >> shift) & 127) | 0x80;
    	else
    		buf[0 - pre + n] =
    			((len >> shift) & 127);
    	n++;
    	shift -= 7;
    }

Change type to signed integer.

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@auriga.com>