Fix crash when writing buffers with nested structs;

Author: bjornbytes

src/api/l_graphics.c

@@ -590,6 +590,7 @@ static uint32_t luax_checkbufferformat(lua_State* L, int index, DataField* field
     lua_getfield(L, -1, "type");
     if (lua_isnil(L, -1)) lua_pop(L, 1), lua_rawgeti(L, -1, 2);
     if (lua_istable(L, -1)) {
+      field->type = ~0u;
       field->fields = fields + *count;
       field->fieldCount = luax_checkbufferformat(L, -1, fields, count, max);
     } else if (lua_type(L, -1) == LUA_TSTRING) {

src/api/l_graphics_buffer.c

@@ -251,7 +251,7 @@ static void luax_checkstruct(lua_State* L, int index, const DataField* structure
 
     if (lua_isnil(L, -1)) {
       memset(data + field->offset, 0, MAX(field->length, 1) * field->stride);
-    } else if (typeComponents[field->type] > 1 && lua_type(L, -1) == LUA_TNUMBER) {
+    } else if (field->length == 0 && field->fieldCount == 0 && typeComponents[field->type] > 1 && lua_type(L, -1) == LUA_TNUMBER) {
       // Need to special case situation where number is provided for vector field,
       // luax_checkbufferdata doesn't handle this properly
       int n = typeComponents[field->type];