fuzz: prevent pcs slice panic and null-base memcpy; ensure capacity before first append

[ghost]

Fixes two crash paths in the new fuzz runner:

std/Build.Fuzz.addEntryPoint: guard pcs.len == 0 (avoid pcs[1..] slice panic) and make logs bounds-safe.

lib/fuzzer.zig:

appendSliceAssumeCapacity: early-return on zero-length and copy into [old_len .. old_len + items.len] to avoid forming a slice off a null base.

start(): ensure capacity before the first append when the corpus is empty (Web UI/coverage path triggers this).

minimal reproduction using zig master: zig init

// src/main.zig
const std = @import("std");
test "fuzz minimal" {
const Ctx = struct { fn testOne(_: @this(), input: []const u8) !void { _ = input; } };
try std.testing.fuzz(Ctx{}, Ctx.testOne, .{});
}

Run: zig build test --fuzz --webui=[::1]:45891.

Before: either a pcs slice panic or a segfault at the fuzzer memcpy (null-base slice).
After: fuzzing runs indefinitely (Ctrl-C to stop).

Note: the ensureTotalCapacity(len) is defensive; a higher-level invariant likely intended the assume-capacity precondition to hold. A TODO comment is left to flag follow-up.

[ifreund]

This isn't the correct fix or even in the correct direction, see #23423 for context.