InternPool: fix segfault in rehashTrackedInsts

The `.empty` map in a shard is weird: it claims to have capacity 1, but
you're not actually allowed to actually use that capacity. That's fine
for the normal insertion algorithm, because it always resizes to a
higher capacity when inserting the initial element. However,
`rehashTrackedInsts` was not aware of this caveat, so sometimes tried to
store to the single element of the `empty` map.

This system exists to avoid an extra branch in the main resizing logic
(since `new_cap = old_cap * 2` only works if the capacity is never
non-zero). However, it's fine for `rehashTrackedInsts` to have an extra
branch to handle this case, since it's literally called once per update.

Author: mlugg

src/InternPool.zig

@@ -319,11 +319,18 @@ pub fn rehashTrackedInsts(
     // We know how big each shard must be, so ensure we have the capacity we need.
     for (ip.shards) |*shard| {
         const want_capacity = if (shard.mutate.tracked_inst_map.len == 0) 0 else cap: {
-            break :cap std.math.ceilPowerOfTwo(u32, shard.mutate.tracked_inst_map.len * 5 / 3) catch unreachable;
+            // We need to return a capacity of at least 2 to make sure we don't have the `Map(...).empty` value.
+            // For this reason, note the `+ 1` in the below expression. This matches the behavior of `trackZir`.
+            break :cap std.math.ceilPowerOfTwo(u32, shard.mutate.tracked_inst_map.len * 5 / 3 + 1) catch unreachable;
         };
         const have_capacity = shard.shared.tracked_inst_map.header().capacity; // no acquire because we hold the mutex
         if (have_capacity >= want_capacity) {
-            @memset(shard.shared.tracked_inst_map.entries[0..have_capacity], .{ .value = .none, .hash = undefined });
+            if (have_capacity == 1) {
+                // The map is `.empty` -- we can't memset the entries, or we'll segfault, because
+                // the buffer is secretly constant.
+            } else {
+                @memset(shard.shared.tracked_inst_map.entries[0..have_capacity], .{ .value = .none, .hash = undefined });
+            }
             continue;
         }
         var arena = arena_state.promote(gpa);