fix bug: []map[string]string type will crash!

[windprog]

Describe the PR
e.g. add cool parser.
if map value is string,it will crash!

Relation issue
e.g. https:/swaggo/swag/pull/118/files

Additional context

test code

type SwagReturn []map[string]string

// @Success 200 {object}  main.SwagReturn	"ok"
func test(){}

result

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x13f0b28]

goroutine 1 [running]:
github.com/swaggo/swag.(*Parser).ParseDefinition(0xc000334140, 0xc00002e95a, 0xa, 0xc000454676, 0x6, 0x0)
/Users/windpro/code/github/swag/parser.go:447 +0x218
github.com/swaggo/swag.(*Parser).parseTypeExpr(0xc000334140, 0xc00002e95a, 0xa, 0x0, 0x0, 0x1596240, 0xc0002f4260, 0x0, 0x0, 0x0, ...)
/Users/windpro/code/github/swag/parser.go:545 +0x27f
github.com/swaggo/swag.(*Parser).parseTypeExpr(0xc000334140, 0xc00002e95a, 0xa, 0x0, 0x0, 0x1596440, 0xc0003164e0, 0x0, 0x0, 0x0, ...)
/Users/windpro/code/github/swag/parser.go:580 +0x717
github.com/swaggo/swag.(*Parser).parseTypeExpr(0xc000334140, 0xc00002e95a, 0xa, 0xc000454660, 0xa, 0x1595c00, 0xc000316510, 0x0, 0x0, 0x0, ...)
/Users/windpro/code/github/swag/parser.go:555 +0x1286
github.com/swaggo/swag.(*Parser).ParseDefinition(0xc000334140, 0xc00002e95a, 0xa, 0xc000454660, 0xa, 0xc0003164b0)
/Users/windpro/code/github/swag/parser.go:447 +0x26f
github.com/swaggo/swag.(*Parser).ParseDefinitions(0xc000334140)
/Users/windpro/code/github/swag/parser.go:426 +0x2f2
github.com/swaggo/swag.(*Parser).ParseAPI(0xc000334140, 0x7ffeefbff4b6, 0x35, 0x7ffeefbff469, 0x46, 0x1, 0x1)
/Users/windpro/code/github/swag/parser.go:99 +0x269
github.com/swaggo/swag/gen.(*Gen).Build(0xc0001c9920, 0x7ffeefbff4b6, 0x35, 0x7ffeefbff469, 0x46, 0x7ffeefbff414, 0x46, 0x7ffeefbff4ff, 0xa, 0x0, ...)
/Users/windpro/code/github/swag/gen/gen.go:30 +0x28e
main.main.func1(0xc000338160, 0x1010100, 0xc000338160)
/Users/windpro/code/github/swag/cmd/swag/main.go:33 +0x18a
github.com/urfave/cli.HandleAction(0x146cd60, 0x15274c8, 0xc000338160, 0xc0002ee000, 0x0)
/Users/windpro/golang/pkg/mod/github.com/urfave/[email protected]/app.go:490 +0xc8
github.com/urfave/cli.Command.Run(0x15097e0, 0x4, 0x0, 0x0, 0xc0003b00c0, 0x1, 0x1, 0x150c96f, 0xe, 0x0, ...)
/Users/windpro/golang/pkg/mod/github.com/urfave/[email protected]/command.go:210 +0x990
github.com/urfave/cli.(*App).Run(0xc0000b2000, 0xc000098180, 0x6, 0x6, 0x0, 0x0)
/Users/windpro/golang/pkg/mod/github.com/urfave/[email protected]/app.go:255 +0x687
main.main()
/Users/windpro/code/github/swag/cmd/swag/main.go:61 +0x4a8

[easonlin404]

LGTM

[easonlin404]

@windprog Thanks for your contributions. 💯