Fix issues when additional metadata isn't available in the first frames

[JeromeMartinez]

e.g. crash with cropdetect filter and several other filters, but could also be wrong metadata in some other corner cases).

Fix #674.

Test build.

[dericed]

seems ok at resolving the issue. could you elaborate on "could also be wrong metadata"?

[JeromeMartinez]

could you elaborate on "could also be wrong metadata"?

In theory, metadata may be not assigned to the right frame instead of crashing.

[dericed]

I have a sample file with cropdetect data from this for each frame:

            <tag key="lavfi.cropdetect.x1" value="0.000000"/>
            <tag key="lavfi.cropdetect.x2" value="320.000000"/>
            <tag key="lavfi.cropdetect.y1" value="0.000000"/>
            <tag key="lavfi.cropdetect.y2" value="240.000000"/>
            <tag key="lavfi.cropdetect.w" value="320.000000"/>
            <tag key="lavfi.cropdetect.h" value="240.000000"/>
            <tag key="lavfi.cropdetect.x" value="0"/>
            <tag key="lavfi.cropdetect.y" value="0"/>

but the plots for x1,x2 and for y1,y2 are with a y-axis of 0-1, although item defines the range as 0-nullptr. In mouse-over preview the values are only either 0 or 1. The plot for x,y has no y-axis at all.

[dericed]