panic: runtime error: invalid memory address or nil pointer dereference

Is go-curl thread safe? I've hit two panics one in goCallProgressFunction other in goCallWriteFunction after making concurrent requests in goroutines.

Code to replicate: https://gist.github.com/joaolsilva/bc4d73122049d6fe5bb9e2d79f81579c

panic: runtime error: invalid memory address or nil pointer dereference
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x20 pc=0x409eb90]

goroutine 424 [running]:
github.com/andelf/go-curl.goCallProgressFunction(0x0, 0x0, 0x0, 0x0, 0x4886c00, 0x0)
/Users/jsilva/go/src/github.com/andelf/go-curl/callback.go:40 +0x40
github.com/andelf/go-curl._cgoexpwrap_b3b701c3ac65_goCallProgressFunction(0x0, 0x0, 0x0, 0x0, 0x4886c00, 0x4886c00)
github.com/andelf/go-curl/_obj/_cgo_gotypes.go:1449 +0x5b
github.com/andelf/go-curl._Cfunc_curl_easy_cleanup(0x4886c00)
github.com/andelf/go-curl/_obj/_cgo_gotypes.go:668 +0x41
github.com/andelf/go-curl.(*CURL).Cleanup.func1(0x4886c00)
/Users/jsilva/go/src/github.com/andelf/go-curl/easy.go:163 +0x60
github.com/andelf/go-curl.(*CURL).Cleanup(0xc4200edae0)
/Users/jsilva/go/src/github.com/andelf/go-curl/easy.go:163 +0x33
panic(0x40bd760, 0x4146c40)
/usr/local/Cellar/go/1.8.3/libexec/src/runtime/panic.go:489 +0x2cf
github.com/andelf/go-curl.goCallProgressFunction(0x0, 0x0, 0x0, 0x0, 0x4886c00, 0x0)
/Users/jsilva/go/src/github.com/andelf/go-curl/callback.go:40 +0x40
github.com/andelf/go-curl._cgoexpwrap_b3b701c3ac65_goCallProgressFunction(0x0, 0x0, 0x0, 0x0, 0x4886c00, 0x0)
github.com/andelf/go-curl/_obj/_cgo_gotypes.go:1449 +0x5b
github.com/andelf/go-curl._Cfunc_curl_easy_perform(0x4886c00, 0x0)
github.com/andelf/go-curl/_obj/_cgo_gotypes.go:795 +0x49
github.com/andelf/go-curl.(*CURL).Perform.func1(0x4886c00, 0x10)
/Users/jsilva/go/src/github.com/andelf/go-curl/easy.go:344 +0x60
github.com/andelf/go-curl.(*CURL).Perform(0xc4200edae0, 0x4e58, 0x40be540)
/Users/jsilva/go/src/github.com/andelf/go-curl/easy.go:344 +0x2e
main.GetURL(0x40dd2d0, 0x43, 0x10, 0x8, 0x0, 0x0, 0x0, 0x0, 0x0)
/Users/jsilva/go/src/curl-crash/curl-crash.go:56 +0x4a1
created by main.main
/Users/jsilva/go/src/curl-crash/curl-crash.go:81 +0x97

If the progress func is removed the crash happens in the write func

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x409ecb1]

goroutine 2434 [running]:
github.com/andelf/go-curl.goCallWriteFunction(0x8faf460, 0x867, 0x8f6fa00, 0xc400000008)
/Users/jsilva/go/src/github.com/andelf/go-curl/callback.go:31 +0x61
github.com/andelf/go-curl._cgoexpwrap_b3b701c3ac65_goCallWriteFunction(0x8faf460, 0x867, 0x8f6fa00, 0x0)
github.com/andelf/go-curl/_obj/_cgo_gotypes.go:1436 +0x72
github.com/andelf/go-curl._Cfunc_curl_easy_perform(0x8f6fa00, 0x0)
github.com/andelf/go-curl/_obj/_cgo_gotypes.go:795 +0x49
github.com/andelf/go-curl.(*CURL).Perform.func1(0x8f6fa00, 0x20)
/Users/jsilva/go/src/github.com/andelf/go-curl/easy.go:344 +0x60
github.com/andelf/go-curl.(*CURL).Perform(0xc422e40460, 0x4e2b, 0x40bba20)
/Users/jsilva/go/src/github.com/andelf/go-curl/easy.go:344 +0x2e
main.GetURL(0x40dd350, 0x43, 0x10, 0x8, 0x0, 0x0, 0x0, 0x0, 0x0)
/Users/jsilva/go/src/curl-crash/curl-crash.go:58 +0x404
created by main.main
/Users/jsilva/go/src/curl-crash/curl-crash.go:83 +0x97

Before crashing it also prints "curl: Couldn't resolve host name". This error doesn't happen if only a request is made at a time.