logging: crash if fmt is not string

closes gh-2516

src/lua/log.lua

@@ -43,6 +43,8 @@ local function say(level, fmt, ...)
         if not stat then
             error(fmt, 3)
         end
+    elseif type(fmt) ~= 'string' then
+        fmt = tostring(fmt)
     end
     local frame = debug.getinfo(3, "Sl")
     local line, file = 0, 'eval'

test/app-tap/logger.test.lua

@@ -48,5 +48,7 @@ debug = nil
 log.info("debug is nil")
 debug = require('debug')
 
+test:ok(log.info(true) == nil, 'check tarantool crash (gh-2516)')
+
 test:check()
 os.exit()