souphttpsrc: reset read_position when reading fails

souphttpsrc: reset read_position when reading fails

souphttpsrc maintains two variables for the position:
 * 'request_position' is where we want to be
 * 'read_position' is where we are
During Normal operations both are updated in sync when data arrives. A seek
changes 'request_position' but not 'read_position'.
When the two positions get out of sync, then a new request is send and the
'Range' header is adjusted to the current 'request_position'.

Without this patch, if reading fails, then the source is destroyed. This
triggers a new request, but the range remains unchanged. As a result, the
old range is used and old data will be read.

Changing the 'read_position' to -1 makes it explicitly different from
'request_position' and as a result the 'Range' header is updated correctly.

https://bugzilla.gnome.org/show_bug.cgi?id=773509