Fix trivial Coverity warnings

  * DLGImporter: use unsigned int instead of unsigned int* in calloc
    (fixes coverity #1004473)

  * GUIScript: Init member var, remove unused code
    (fixes coverity #1004490, #1004564)

  * MUSImporter: Init member var and change strncpy() to strlcpy()
    (fixes coverity #1004327, #1004328, #1004568)

  * VLCPlayer/VideoContext: limit plane index to 0..2
    (coverity #1004436)