Aim the truncated-directory test at the raising path

A record whose name is cut short already reported; the raise needed a
zero-length name and a directory offset pointing into the trailing
comment. Build that case instead.

tests/archive_spec.lua

@@ -322,8 +322,8 @@ t.describe('archive.extract zip', function()
 end)
 
 t.describe('archive.extract zip malformed directory', function()
-  -- A central directory record that stops before its extra-field and comment
-  -- lengths used to raise instead of reporting.
+  -- A central directory record that stops short of its own header used to
+  -- raise instead of reporting.
   local function truncate_cd(keep)
     local bytes = zipf({ { name = 'root/a.lua', data = 'hi' } })
     local cd = bytes:find('PK\1\2', 1, true)
@@ -331,8 +331,14 @@ t.describe('archive.extract zip malformed directory', function()
     return bytes:sub(1, cd + keep - 1) .. bytes:sub(eocd)
   end
 
-  t.it('reports a record cut off before its length fields', function()
-    local ok, err = extract(truncate_cd(30))
+  t.it('reports a record cut off before its extra-field lengths', function()
+    -- The directory offset is attacker-controlled, so point it at a record
+    -- planted in the trailing comment: a zero name length lets the parse run
+    -- cleanly up to the extra-field and comment lengths, which is where it
+    -- used to raise.
+    local comment = 'PK\1\2' .. string.rep('\0', 26)
+    local bytes = zipf({ { name = 'root/a.lua', data = 'hi' }, comment = comment })
+    local ok, err = extract(patch32(bytes, #bytes - #comment - 21 + 16, #bytes - #comment))
     t.falsy(ok)
     t.matches('truncated central directory', err)
   end)