rtspconnection: Fix GError set over the top of a previous GError

The function fill_bytes could sometimes return a value greater than zero
and in the same time set the GError.

Function read_bytes calls fill_bytes in a while loop. In the special
case above it would call fill_bytes with error already set.
Thus resulting in "GError set over the top of a previous GError".

Solved this by clearing GError when return value is greater than zero.
Actions are taken depending on error type by caller of read_bytes. Eg.
with EWOULDBLOCK gst_rtsp_source_dispatch_read will try to read the
missing bytes again (GST_RTSP_EINTR )

#445
8 jobs from master in 34 minutes and 30 seconds (queued for 13 seconds)
Status Job ID Name Coverage
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passed #124004
gst indent

00:01:06

passed #124003
manifest

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build fedora x86_64

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  Test
passed #124006
check fedora

00:04:48

 
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build android arm64 api28

00:04:06

passed #124009
build cerbero cross win32

00:14:56

passed #124010
build cerbero cross win64

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passed #124008
build cerbero fedora x86_64

00:05:49