Qt4: fix invalid pointer on unknown V4L2 control type (fix #5391)
Qt4: fix invalid pointer on unknown V4L2 control type (fix #5391)

var_Change(VLC_VAR_GETTEXT) fails if there is no text. In that case,
the value is undefined.

xkfz007/vlc,shyamalschandra/vlc,xkfz007/vlc,shyamalschandra/vlc,shyamalschandra/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,vlc-mirror/vlc,vlc-mirror/vlc-2.1,krichter722/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc,shyamalschandra/vlc,krichter722/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,shyamalschandra/vlc,krichter722/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,shyamalschandra/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,krichter722/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.1,krichter722/vlc,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,jomanmuk/vlc-2.2,krichter722/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2