check input ranges for sanity (index out of range)

branan@gfz:~/gnss-tools/dateconv$ bin/dateconv convert 10000 10000 panic: runtime error: index out of range [-7]

goroutine 1 [running]: git.gfz-potsdam.de/gnss/tools/dateconv/dateconv.ydToYmd(0x40c3880000000000?, 0x7ffdf231e4c0?) /home/branan/gnss-tools/dateconv/dateconv/dateconv.go:1189 +0x265 git.gfz-potsdam.de/gnss/tools/dateconv/dateconv.mjdToDate(0x7ffdf231e4c0?, 0x5?) /home/branan/gnss-tools/dateconv/dateconv/dateconv.go:1126 +0x12d git.gfz-potsdam.de/gnss/tools/dateconv/dateconv.(*Conversion).getDatetimeFromMjd(0xc0000740a0) /home/branan/gnss-tools/dateconv/dateconv/dateconv.go:885 +0x65 git.gfz-potsdam.de/gnss/tools/dateconv/dateconv.(*Conversion).callAllUntilMatch(0x280?) /home/branan/gnss-tools/dateconv/dateconv/dateconv.go:673 +0x13c git.gfz-potsdam.de/gnss/tools/dateconv/dateconv.(*Conversion).determineFormat(0xc00006a0c0?) /home/branan/gnss-tools/dateconv/dateconv/dateconv.go:605 +0x505 git.gfz-potsdam.de/gnss/tools/dateconv/dateconv.(*Conversion).run(0xc0000740a0) /home/branan/gnss-tools/dateconv/dateconv/dateconv.go:72 +0x25 git.gfz-potsdam.de/gnss/tools/dateconv/dateconv.NewConversion({0x7ffdf231e4c0, 0x5}, {0x0, 0x0}, {0x0?, 0x0?}) /home/branan/gnss-tools/dateconv/dateconv/dateconv.go:52 +0xee git.gfz-potsdam.de/gnss/tools/dateconv/cmd.glob..func1(0xc000114900?, {0xc000028200?, 0x4?, 0x5bd8cf?}) /home/branan/gnss-tools/dateconv/cmd/convert.go:25 +0x98 github.com/spf13/cobra.(*Command).execute(0x734b20, {0xc0000281b0, 0x1, 0x1}) /home/branan/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:987 +0xaa3 github.com/spf13/cobra.(*Command).ExecuteC(0x734e00) /home/branan/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1115 +0x3ff github.com/spf13/cobra.(*Command).Execute(...) /home/branan/go/pkg/mod/github.com/spf13/cobra@v1.8.0/command.go:1039 git.gfz-potsdam.de/gnss/tools/dateconv/cmd.Execute() /home/branan/gnss-tools/dateconv/cmd/root.go:26 +0x1a main.main() /home/branan/gnss-tools/dateconv/main.go:8 +0xf