glupload: prevent segfault when updating caps

When the glupload element renegotiates the caps, set_caps will reset the
method_impl to NULL, but the method will be kept. transform_caps tries
to use the method_impl to transform the caps, because a method is set,
but will segfault.
17 jobs for glupload in 94 minutes and 27 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Preparation
passed #233285
gst indent

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passed #233284
manifest

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  Build
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build fedora x86_64

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build nodebug fedora x86_64

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  Test
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check fedora

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integration testsuites fedora

00:11:57

 
  Full Builds
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build cerbero android universal

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build cerbero cross win32

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passed #233293
build cerbero cross win64

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passed #233290
build cerbero fedora x86_64

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  Full Tests
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valgrind bad

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passed #233294
valgrind base

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passed #233413
valgrind good

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passed #233296
valgrind ugly

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failed #233391
valgrind good

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failed #233295
valgrind good

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  Apps
passed #233298
android universal examples

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