Parsing the regexp /\A{/ causes uses an uninitialized value because it
tries to parse it as a range quantifier, so it reads the character after
the closing curly bracket. This is using uninitialized values because
prism strings are not null terminated. This can be seen in the Valgrind
output:

    ==834710== Conditional jump or move depends on uninitialised value(s)
    ==834710==    at 0x5DA010: pm_regexp_parse_range_quantifier (regexp.c:163)
    ==834710==    by 0x5DA010: pm_regexp_parse_quantifier (regexp.c:243)
    ==834710==    by 0x5DAD69: pm_regexp_parse_expression (regexp.c:738)
    ==834710==    by 0x5DAD69: pm_regexp_parse_pattern (regexp.c:761)
    ==834710==    by 0x5DAD69: pm_regexp_parse (regexp.c:773)
    ==834710==    by 0x5A2EE7: parse_regular_expression_named_captures (prism.c:20886)
    ==834710==    by 0x5A2EE7: parse_expression_infix (prism.c:21388)
    ==834710==    by 0x5A5FA5: parse_expression (prism.c:21804)
    ==834710==    by 0x5A64F3: parse_statements (prism.c:13858)
    ==834710==    by 0x5A9730: parse_program (prism.c:22011)
    ==834710==    by 0x576F0D: parse_input_success_p (extension.c:1062)
    ==834710==    by 0x576F0D: parse_success_p (extension.c:1084)

This commit adds checks for the end of the string to
pm_regexp_parse_range_quantifier.

https://github.com/ruby/prism/commit/be6cbc23ef

https://github.com/ruby/prism/commit/6f886be0a4

https://github.com/ruby/prism/commit/5050dfbe70

You can't encoding a string with more than one encoding so ensure that
the last one wins.

https://github.com/ruby/prism/commit/4182c988c9