Compare with ffi.NULL instead of nil

This is required for standard Lua, see
https://github.com/facebookarchive/luaffifb#pointer-comparison

src/vips/Image_methods.lua

@@ -122,8 +122,8 @@ end
 
 function Image.find_load(filename)
     local name = vips_lib.vips_foreign_find_load(filename)
-    if name == nil then
-        return nil
+    if name == ffi.NULL then
+        return ffi.NULL
     else
         return ffi.string(name)
     end
@@ -134,7 +134,7 @@ function Image.new_from_file(vips_filename, ...)
     local options = to_string_copy(vips_lib.vips_filename_get_options(vips_filename))
 
     local name = Image.find_load(filename)
-    if name == nil then
+    if name == ffi.NULL then
         error(verror.get())
     end
 
@@ -143,7 +143,7 @@ end
 
 function Image.find_load_buffer(data)
     local name = vips_lib.vips_foreign_find_load_buffer(data, #data)
-    if name == nil then
+    if name == ffi.NULL then
         return nil
     else
         return ffi.string(name)
@@ -152,7 +152,7 @@ end
 
 function Image.new_from_buffer(data, options, ...)
     local name = Image.find_load_buffer(data)
-    if name == nil then
+    if name == ffi.NULL then
         error(verror.get())
     end
 
@@ -163,7 +163,7 @@ function Image.new_from_memory_ptr(data, size, width, height, bands, format)
     local format_value = gvalue.to_enum(gvalue.band_format_type, format)
     local vimage = vips_lib.vips_image_new_from_memory(data, size,
             width, height, bands, format_value)
-    if vimage == nil then
+    if vimage == ffi.NULL then
         error(verror.get())
     end
     return Image.new(vimage)
@@ -361,7 +361,7 @@ end
 
 function Image_method:copy_memory()
     local vimage = vips_lib.vips_image_copy_memory(self.vimage)
-    if vimage == nil then
+    if vimage == ffi.NULL then
         error(verror.get())
     end
     return Image.new(vimage)
@@ -373,7 +373,7 @@ function Image_method:write_to_file(vips_filename, ...)
     local filename = to_string_copy(vips_lib.vips_filename_get_filename(vips_filename))
     local options = to_string_copy(vips_lib.vips_filename_get_options(vips_filename))
     local name = vips_lib.vips_foreign_find_save(filename)
-    if name == nil then
+    if name == ffi.NULL then
         error(verror.get())
     end
 
@@ -384,7 +384,7 @@ end
 function Image_method:write_to_buffer(format_string, ...)
     local options = to_string_copy(vips_lib.vips_filename_get_options(format_string))
     local name = vips_lib.vips_foreign_find_save_buffer(format_string)
-    if name == nil then
+    if name == ffi.NULL then
         error(verror.get())
     end
 

src/vips/gvalue.lua

@@ -172,7 +172,7 @@ gvalue.get = function(gv)
 
         local cstr = vips_lib.vips_enum_nick(gtype_raw, enum_value)
 
-        if cstr == nil then
+        if cstr == ffi.NULL then
             error("value not in enum")
         end
 
@@ -182,7 +182,7 @@ gvalue.get = function(gv)
     elseif gtype == gvalue.gstr_type then
         local cstr = gobject_lib.g_value_get_string(gv)
 
-        if cstr ~= nil then
+        if cstr ~= ffi.NULL then
             result = ffi.string(cstr)
         else
             result = nil

src/vips/voperation.lua

@@ -144,7 +144,7 @@ voperation.call = function(name, string_options, ...)
     local call_args = { ... }
 
     local vop = vips_lib.vips_operation_new(name)
-    if vop == nil then
+    if vop == ffi.NULL then
         error("no such operation\n" .. verror.get())
     end
     vop = vop:new()
@@ -234,7 +234,7 @@ voperation.call = function(name, string_options, ...)
     end
 
     local vop2 = vips_lib.vips_cache_operation_build(vop)
-    if vop2 == nil then
+    if vop2 == ffi.NULL then
         error("unable to call " .. name .. "\n" .. verror.get())
     end
     vop = vop2:new()