Only check REST arguments if the file does not exist

[AdamMainsTL]

Description

Fix for #4084. Will now check if the given filename exists before checking for REST parameters.

Tests

I did not look into test cases for this, but I ran the following command before and after the change:

Before:

bin % ./oiiotool \?.png -o \?-output.png
oiiotool ERROR: read : "?.png": ImageInput::create() called with malformed filename
Full command line was:
> oiiotool ?.png -o ?-output.png

After:

bin % ./oiiotool \?.png -o \?-output.png

No output, ?-output.png created.

Checklist:

• I have read the contribution guidelines.
• I have updated the documentation, if applicable.
• I have ensured that the change is tested somewhere in the testsuite
  (adding new test cases if necessary).
• If I added or modified a C++ API call, I have also amended the
  corresponding Python bindings (and if altering ImageBufAlgo functions, also
  exposed the new functionality as oiiotool options).
• My code follows the prevailing code style of this project. If I haven't
  already run clang-format before submitting, I definitely will look at the CI
  test that runs clang-format and fix anything that it highlights as being
  nonconforming.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: AdamMainsTL (f206e09)

[lgritz]

The solution looks right to me!

Can you please amend to fix the DCO signoff? The easiest way would be

git commit --amend -s
(just save, you don't need to change anything, the -s adds the sign-off)
git push origin fix/imageinput-rest-check  --force

[AdamMainsTL]

Should be done!

[lgritz]

LGTM, thanks!

And I will backport to the release branch for the Jan 1 patch release.