nil dereference in reading metadata

[osklyar]

In func (dataset *Dataset) Metadata(domain string) []string (gdal.go:687) when the execution reaches the end of the list it panics with nil dereference. For some reason I have observed this behavior on OSX only, but not on linux:

➜  Downloads hdf2tiff HDF4_EOS:EOS_GRID:"MYD10A1.A2008006.h19v02.006.2016142113721.hdf":MOD_Grid_Snow_500m:NDSI_Snow_Cover foo.tiff
unexpected fault address 0xb01dfacedebac1e
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x1 addr=0xb01dfacedebac1e pc=0x4005d6e]

goroutine 1 [running]:
runtime.throw(0x40ecae4, 0x5)
	/usr/local/go/src/runtime/panic.go:1117 +0x72 fp=0xc0000b3af0 sp=0xc0000b3ac0 pc=0x40370d2
runtime.sigpanic()
	/usr/local/go/src/runtime/signal_unix.go:741 +0x276 fp=0xc0000b3b28 sp=0xc0000b3af0 pc=0x404bf36
indexbytebody(0x554c4f4341544144, 0xebc, 0x0, 0xc0000b3b78, 0xebc, 0x0, 0x554c4f4341544144, 0x554c4f4341544144, 0xebc, 0xc0000b3bd0, ...)
	/usr/local/go/src/internal/bytealg/indexbyte_amd64.s:124 +0xce fp=0xc0000b3b30 sp=0xc0000b3b28 pc=0x4005d6e
runtime.findnull(0x554c4f4341544144, 0x0)
	/usr/local/go/src/runtime/string.go:438 +0x87 fp=0xc0000b3b88 sp=0xc0000b3b30 pc=0x4051d27
runtime.gostring(0x554c4f4341544144, 0x0, 0x0)
	/usr/local/go/src/runtime/string.go:320 +0x2f fp=0xc0000b3be0 sp=0xc0000b3b88 pc=0x4065daf
github.com/lukeroth/gdal._Cfunc_GoString(...)
	_cgo_gotypes.go:3002
github.com/lukeroth/gdal.(*Dataset).Metadata(0xc0000b3d88, 0x0, 0x0, 0x0, 0x0, 0x0)
	/Users/osklyar/Library/Go/pkg/mod/github.com/osklyar/gdal@v0.0.0-20210506223057-2fecd07bf2d8/gdal.go:687 +0x109 fp=0xc0000b3c78 sp=0xc0000b3be0 pc=0x40b3929
github.com/nordicsense/modis/dataset.Open(0x7ffeefbffaf9, 0x62, 0x40eb200, 0xc00008c101, 0xc000082480, 0xc0000486c0)
	/Users/osklyar/Library/Go/pkg/mod/github.com/nordicsense/modis@v0.0.0-20210509214422-ca11fe4bf0d6/dataset/file.go:46 +0x45e fp=0xc0000b3e58 sp=0xc0000b3c78 pc=0x40b833e
main.convert(0x7ffeefbffaf9, 0x62, 0x7ffeefbffb5c, 0x8, 0x0, 0x0)
	/Users/osklyar/src/github.com/nordicsense/lstproxy/hdf2tiff/hdf2tiff.go:18 +0x70 fp=0xc0000b3f38 sp=0xc0000b3e58 pc=0x40ba150
main.main()
	/Users/osklyar/src/github.com/nordicsense/lstproxy/hdf2tiff/hdf2tiff.go:12 +0x67 fp=0xc0000b3f88 sp=0xc0000b3f38 pc=0x40ba067
runtime.main()
	/usr/local/go/src/runtime/proc.go:225 +0x256 fp=0xc0000b3fe0 sp=0xc0000b3f88 pc=0x40398f6
runtime.goexit()
	/usr/local/go/src/runtime/asm_amd64.s:1371 +0x1 fp=0xc0000b3fe8 sp=0xc0000b3fe0 pc=0x4068e61

The patch #70 is ready and tested and I will submit a PR separately. It should work equally on linux as it only adds another nil check.