#797 throw ImageFormatException when no StartOfFrame marker is found

[jongleur1983]

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Description

Fixes #797:

For jpg images where we can't find a valid SOF (start of frame) marker we now throw an ImageFormatException reporting that fact instead of a NullReferenceException.

Added the failing image from the issue for a test case.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@db0fc0d). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #801   +/-   ##
=========================================
  Coverage          ?   88.79%           
=========================================
  Files             ?     1012           
  Lines             ?    43313           
  Branches          ?     3124           
=========================================
  Hits              ?    38459           
  Misses            ?     4156           
  Partials          ?      698

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)
...arp.Tests/Formats/Jpg/JpegDecoderTests.Baseline.cs	86.66% <100%> (ø)
src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs	87.75% <100%> (ø)

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@db0fc0d). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #801   +/-   ##
=========================================
  Coverage          ?   88.79%           
=========================================
  Files             ?     1012           
  Lines             ?    43313           
  Branches          ?     3124           
=========================================
  Hits              ?    38459           
  Misses            ?     4156           
  Partials          ?      698

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)
...arp.Tests/Formats/Jpg/JpegDecoderTests.Baseline.cs	86.66% <100%> (ø)
src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs	87.75% <100%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update db0fc0d...59d76a0. Read the comment docs.

[JimBobSquarePants]

Awesome, thanks!