Add sanity check in infix handling, fixes R#3949 (#3951)
Add sanity check in infix handling, fixes R#3949 (#3951)

Before this, an "infix:(&)" (note the missing < > there) would cause a:

    $ raku -e 'infix:(&)'
    This type (QAST::WVal) does not support positional operations

crash without any indication on *where* in the code the actual error was.
After this, it will cause a correct compilation error:

    $ raku -e 'infix:(&)'
    ===SORRY!=== Error while compiling -e
    You can't adverb :(&)
    at -e:1
    ------> infix:(&)⏏<EOL>
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