DDX/RandR: When positioning outputs assume width/height = 0 if no modes exist.
When a user specifies the position of an output for which no modes exist (for whatever reason) assume that the width and height of this output is 0. The result will be the same as if this output isn't taken into consideration at all and thus should be sane. It will prevent a segfault when trying to determine the width and height of a non-existent mode.
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