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

lib/fuzzer.zig

@@ -279,6 +279,7 @@ const Fuzzer = struct {
             const len = rng.uintLessThanBiased(usize, 200);
             const slice = try gpa.alloc(u8, len);
             rng.bytes(slice);
+            f.input.ensureTotalCapacity(len) catch @panic("mmap file resize failed"); // TODO: Not clear yet what causes the capacity assumption to be false.
             f.input.appendSliceAssumeCapacity(slice);
             try f.corpus.append(gpa, .{
                 .bytes = slice,
@@ -600,6 +601,7 @@ pub const MemoryMappedList = struct {
     /// Append the slice of items to the list.
     /// Asserts that the list can hold the additional items.
     pub fn appendSliceAssumeCapacity(l: *MemoryMappedList, items: []const u8) void {
+        if (items.len == 0) return;
         const old_len = l.items.len;
         const new_len = old_len + items.len;
         assert(new_len <= l.capacity);

lib/std/Build/Fuzz.zig

@@ -422,6 +422,10 @@ fn addEntryPoint(fuzz: *Fuzz, coverage_id: u64, addr: u64) error{ AlreadyReporte
     const coverage_map = fuzz.coverage_files.getPtr(coverage_id).?;
     const header: *const abi.SeenPcsHeader = @ptrCast(coverage_map.mapped_memory[0..@sizeOf(abi.SeenPcsHeader)]);
     const pcs = header.pcAddrs();
+    if (pcs.len == 0) {
+        log.err("no program counters recorded for unit test (coverage_id=0x{x}); addr=0x{x}", .{ coverage_id, addr });
+        return error.AlreadyReported;
+    }
 
     // Since this pcs list is unsorted, we must linear scan for the best index.
     const index = i: {