[cling] meta cmd .x should not crash if no file is given

[jiangyilism]

Changes or fixes:

On Ubuntu 22.04 , execute standalone cling binary. In cmd prompt execute .x meta command without giving a filename, cling crashes.

The proposed solution is to directly reject the .x command without giving clear diagnostics. This is not user friendly but this is currently how MetaParser.cpp handles all parsing/input error. At least it prevents crashing.

Checklist:

• [X] tested changes locally
• [ ] updated the docs (if necessary)

[phsft-bot]

Can one of the admins verify this patch?

[jiangyilism]

@phsft-bot build

[jiangyilism]

@Axel-Naumann , @eguiraud Could anyone help reviewing this patch?

[Axel-Naumann]

Sorry for the slow response. What's the behavior / output when running .x without filename once this PR is applied? If there is absolutely no diagnostic, could you maybe add one as part of this PR? (If not then I will merge this as is - it's still better than crashing!)

[jiangyilism]

Sorry for the slow response. What's the behavior / output when running .x without filename once this PR is applied? If there is absolutely no diagnostic, could you maybe add one as part of this PR? (If not then I will merge this as is - it's still better than crashing!)

After this patch, the outputs are as follows:

[cling]$ .x
input_line_3:2:2: error: expected expression
 .x
 ^