jpegdec: don't overwrite the last valid line

If the the height is not a multiple of the macro block size then the memory
of the last line is reused for all extra lines. This is no problem if the
last line is duplicated properly. However, if the extra lines are not
initialized properly during encoding, then the last visible line is
overwritten with undefined data.
Use a extra buffer to avoid this problem.
14 jobs for jpeg-overwrite in 51 minutes and 56 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #293544
gst indent

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passed #293543
manifest

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  Build
passed #293545
build fedora x86_64

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passed #293546
build nodebug fedora x86_64

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  Test
passed #293547
check fedora

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passed #293548
integration testsuites fedora

00:12:07

 
  Full Builds
passed #293550
build cerbero android universal

00:11:55

passed #293551
build cerbero cross win32

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passed #293552
build cerbero cross win64

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passed #293549
build cerbero fedora x86_64

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  Full Tests
passed #293554
valgrind bad

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passed #293555
valgrind ges

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passed #293553
valgrind good

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  Apps
passed #293556
android universal examples

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