curlhttpsrc: set BUFFER_OFFSET when creating GstBuffer

To make curlhttpsrc behave more like souphttpsrc, set the
BUFFER_OFFSET in its output buffers to match the segment
start. This means that in a HTTP RANGE request, the BUFFER_OFFSET
will match the value in the RANGE request.
8 jobs for issue876-curlhttpsrc-leaks in 36 minutes and 34 seconds (queued for 58 seconds)
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passed #126519
gst indent

00:00:17

passed #126518
manifest

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  Build
passed #126520
build fedora x86_64

00:04:41

 
  Test
passed #126521
check fedora

00:03:31

 
  Full Builds
passed #126522
build android arm64 api28

00:03:26

passed #126524
build cerbero cross win32

00:09:38

passed #126525
build cerbero cross win64

00:06:46

passed #126523
build cerbero fedora x86_64

00:07:54