[2.x] JPEG: Handle errors when saving with libjpeg

[smcv]

• JPEG: Factor out the middle of IMG_SaveJPG_RW_jpeglib

  No functional change intended. We'll need to use setjmp() in this function in a subsequent commit, so ensure that its state doesn't include any local variables that are used both "above" and "below" the stack level at which we will call setjmp().

• JPEG: Add error-recovery when saving with libjpeg

  Because we have set up libjpeg to use my_error_exit, we need to call setjmp() before the first time it might possibly call longjmp(). Otherwise, on error we'll do a non-local goto to an uninitialized pointer and crash.

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SDL2 version of #431, which should be merged first (but this is the version that I have actually tested). Please cherry-pick to release-2.8.x if accepted.

[slouken]

Merged and cherry-picked to release-2.8.x