string-to-unibyte in image-jpeg-p

[Thien-Thi Nguyen]

One of the last ‘string-to-unibyte’ (deprecated for a while)
calls is in ‘image-jpeg-p’:

 (defun image-jpeg-p (data)
   "..."
   (setq data (ignore-errors (string-to-unibyte data)))
   ...)

We can simulate ‘string-to-unibyte’ (except for error message
particulars) with ‘FAKE-string-to-unibyte’, which uses the
modern ‘encode-coding-string’:

 (defun FAKE-string-to-unibyte (s)
   (let ((tem (encode-coding-string s 'binary)))
     (when (string-match-p "\xc2" tem)
       (error "badness"))
     tem))

However, the original call ignores errors, so this:

 (setq data (ignore-errors (FAKE-string-to-unibyte data)))

can be simplified to this:

 (setq data (FAKE/NOERROR-string-to-unibyte data))

by defining:

 (defun FAKE/NOERROR-string-to-unibyte (s)
   (let ((tem (encode-coding-string s 'binary)))
     (unless (string-match-p "\xc2" tem)
       tem)))

The last step is to simply inline the definition:

 (setq data (let ((tem (encode-coding-string data 'binary)))
              (unless (string-match-p "\xc2" tem)
                tem)))

My questions are:
- Is my reasoning correct?  In particular, i'd like to confirm
  that the "error" detection via "\xc2" is a valid strategy.
- If so, which branch gets the change, ‘emacs-26’ or ‘master’?
- If not, what am i missing?

-- 
Thien-Thi Nguyen -----------------------------------------------
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   (pcase (context query)
     (`(technical ,ml) (correctp ml))
     ...))                              748E A0E8 1CB8 A748 9BFA
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