build: panic when passing invalid path to tracy

[kubkon]

Zig Version

0.14.0-dev.423+79352635b

Steps to Reproduce and Observed Behavior

$ ./stage3/bin/zig build -p stage4 -Denable-llvm -Doptimize=ReleaseFast -Dstrip=false -Dtracy=/Users/kubkon/dev/tracy/public
thread 8778209 panic: index out of bounds: index 0, len 0
/Users/kubkon/dev/zig/lib/std/Build/Step.zig:549:103: 0x102fa726f in zigProcessUpdate (build)
                    const prefix_index: std.zig.Server.Message.PathPrefix = @enumFromInt(prefixed_path[0] - 1);
                                                                                                      ^
/Users/kubkon/dev/zig/lib/std/Build/Step.zig:441:40: 0x102fa992f in evalZigProcess (build)
    const result = try zigProcessUpdate(s, zp, watch);
                                       ^
/Users/kubkon/dev/zig/lib/std/Build/Step/Compile.zig:1752:54: 0x102f00c6f in make (build)
    const maybe_output_bin_path = step.evalZigProcess(
                                                     ^
/Users/kubkon/dev/zig/lib/std/Build/Step.zig:231:13: 0x102ed72d3 in make (build)
    s.makeFn(s, options) catch |err| switch (err) {
            ^
/Users/kubkon/dev/zig/lib/compiler/build_runner.zig:1046:31: 0x102e7dc9f in workerMakeOneStep (build)
    const make_result = s.make(.{
                              ^
/Users/kubkon/dev/zig/lib/std/Thread/Pool.zig:115:39: 0x102e7e5e7 in runFn (build)
            @call(.auto, func, closure.arguments);
                                      ^
/Users/kubkon/dev/zig/lib/std/Thread/Pool.zig:291:32: 0x102f49ecb in worker (build)
            run_node.data.runFn(&run_node.data, id);
                               ^
/Users/kubkon/dev/zig/lib/std/Thread.zig:409:13: 0x102ed5c3f in callFn__anon_16536 (build)
            @call(.auto, f, args);
            ^
/Users/kubkon/dev/zig/lib/std/Thread.zig:678:30: 0x102e7d2ab in entryFn (build)
                return callFn(f, args_ptr.*);
                             ^
???:?:?: 0x188892f93 in ??? (libsystem_pthread.dylib)
???:?:?: 0x261a80018888dd33 in ??? (???)
error: the following build command crashed:
/Users/kubkon/dev/zig/.zig-cache/o/5b4a0393f7f8ce18b6c3d95edfc42e9e/build /Users/kubkon/dev/zig/build/stage4/bin/zig /Users/kubkon/dev/zig/lib /Users/kubkon/dev/zig /Users/kubkon/dev/zig/.zig-cache /Users/kubkon/.cache/zig --seed 0x8241ce26 -Zc50781c420ca5a9d -p stage6 -Denable-llvm -Doptimize=ReleaseFast -Dstrip=false -Dtracy=/Users/kubkon/dev/tracy/public

Note I mistakenly explicitly included public subpath in tracy path which is not expected as build.zig will prepend that for us, however, it should still not panic.

Expected Behavior

A meaningful error.