fix(rpc): "grid_line" event parsing crashes (#25581)

refactor: use a more idiomatic loop to iterate over the cells

There are two cases in which the following assertion would fail:
```c
assert(g->icell < g->ncells);
```

1. If `g->ncells = 0`. Update this to be legal.
2. If an EOF is reached while parsing `wrap`. In this case, the unpacker
   attempts to resume from `cells`, which is a bug. Create a new state
   for parsing `wrap`.

Reference: https://neovim.io/doc/user/ui.html#ui-event-grid_line

test/unit/msgpack_spec.lua

@@ -0,0 +1,75 @@
+local helpers = require('test.unit.helpers')(after_each)
+local cimport = helpers.cimport
+local itp = helpers.gen_itp(it)
+local lib = cimport('./src/nvim/msgpack_rpc/unpacker.h', './src/nvim/memory.h')
+local ffi = helpers.ffi
+local eq = helpers.eq
+local to_cstr = helpers.to_cstr
+
+--- @class Unpacker
+--- @field read_ptr ffi.cdata*
+--- @field read_size number
+
+--- @alias Unpacker* table<number, Unpacker>
+--- @return Unpacker* unpacker `unpacker[0]` to dereference
+local function make_unpacker()
+  return ffi.gc(ffi.cast('Unpacker*', lib.xcalloc(1, ffi.sizeof('Unpacker'))), function(unpacker)
+    lib.unpacker_teardown(unpacker, nil, nil)
+    lib.xfree(unpacker)
+  end)
+end
+
+--- @param unpacker Unpacker*
+--- @param data string
+--- @param size number? *default: data:len()*
+local function unpacker_goto(unpacker, data, size)
+  unpacker[0].read_ptr = to_cstr(data)
+  unpacker[0].read_size = size or data:len()
+end
+
+--- @param unpacker Unpacker*
+--- @return boolean
+local function unpacker_advance(unpacker)
+  return lib.unpacker_advance(unpacker)
+end
+
+describe('msgpack', function()
+  describe('unpacker', function()
+    itp('does not crash when paused between `cells` and `wrap` params of `grid_line` #25184', function()
+      -- [kMessageTypeNotification, "redraw", [
+      --   ["grid_line",
+      --     [2, 0, 0, [[" " , 0, 77]], false]
+      --   ]
+      -- ]]
+      local payload =
+        '\x93\x02\xa6\x72\x65\x64\x72\x61\x77\x91\x92\xa9\x67\x72\x69\x64\x5f\x6c\x69\x6e\x65\x95\x02\x00\x00\x91\x93\xa1\x20\x00\x4d\xc2'
+
+      local unpacker = make_unpacker()
+      lib.unpacker_init(unpacker)
+
+      unpacker_goto(unpacker, payload, payload:len() - 1)
+      local finished = unpacker_advance(unpacker)
+      eq(finished, false)
+
+      unpacker[0].read_size = unpacker[0].read_size + 1
+      finished = unpacker_advance(unpacker)
+      eq(finished, true)
+    end)
+
+    itp('does not crash when parsing grid_line event with 0 `cells` #25184', function()
+      local unpacker = make_unpacker()
+      lib.unpacker_init(unpacker)
+
+      unpacker_goto(unpacker,
+        -- [kMessageTypeNotification, "redraw", [
+        --   ["grid_line",
+        --     [2, 0, 0, [], false]
+        --   ]
+        -- ]]
+        '\x93\x02\xa6\x72\x65\x64\x72\x61\x77\x91\x92\xa9\x67\x72\x69\x64\x5f\x6c\x69\x6e\x65\x95\x02\x00\x00\x90\xc2'
+      )
+      local finished = unpacker_advance(unpacker)
+      eq(finished, true)
+    end)
+  end)
+end)