[zig] fix fuzz crash

[lukewilliamboswell]

This debug assertion was causing the parser to crash. Removing it seems to resolve the issue.

@gamebox does this look ok?

$ zig build snapshot -- src/snapshots/fuzz_crash/fuzz_crash_018.txt
thread 4022156 panic: reached unreachable code
/Users/luke/zig-macos-aarch64-0.14.0/lib/std/debug.zig:522:14: 0x104f47c93 in assert (snapshot)
    if (!ok) unreachable; // assertion failure
             ^
/Users/luke/Documents/GitHub/roc/src/check/parse/IR.zig:989:33: 0x10503eb3b in addPattern (snapshot)
                std.debug.assert(a.patterns.span.len > 1);
                                ^
/Users/luke/Documents/GitHub/roc/src/check/parse/Parser.zig:1177:33: 0x10506cb93 in parsePattern (snapshot)
    return self.store.addPattern(.{ .alternatives = .{
                                ^
/Users/luke/Documents/GitHub/roc/src/check/parse/Parser.zig:1187:29: 0x105084e73 in parsePatternWithAlts (snapshot)
    return self.parsePattern(.alternatives_allowed);

<snipped...>

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