fix escaped backslash handling in minify_string

[nabhan06]

minify_string only treats a backslash as the start of an escape when the next byte is a double quote, so a string value that ends in an escaped backslash (for example the value "\") is misread: the second backslash and the real closing quote are taken as an escaped quote and consumed, and the scanner keeps going past the end of the string. Everything after it, including comments and whitespace that minify is meant to strip, is then copied verbatim until the next quote. For input like {"p":"\" /* c */ , "q":1} the minified output keeps the comment and no longer parses.

Treat any non-NUL byte after a backslash as an escaped character so the pair is copied and skipped together. Added a regression test in tests/minify_tests.c covering an escaped backslash right before the closing quote.