tokenizer: Fix index-out-of-bounds on string_literal_backslash right before EOF

[squeek502]

Before this fix, the added test would fail with something like:

thread 3287298 panic: index out of bounds
/home/ryan/Programming/zig/zig/build/lib/zig/std/zig/tokenizer.zig:408:34: 0x21511f in std.zig.tokenizer.Tokenizer.next (fuzz-debug)
            const c = self.buffer[self.index];
                                 ^
/home/ryan/Programming/zig/zig/build/lib/zig/std/zig/parse.zig:24:37: 0x20af60 in std.zig.parse.parse (fuzz-debug)
        const token = tokenizer.next();
                                    ^

This was found by fuzzing the tokenizer with https://github.com/squeek502/zig-std-lib-fuzzing

(this is just the beginning, there are lots more tokenizer crashes that have been found)

[andrewrk]

Thanks!