fix: ensure mime box is large enough to read

[eric]

Fix the panic found by fuzzing:

--- FAIL: FuzzDecodeBox (478.19s)
    --- FAIL: FuzzDecodeBox (0.00s)
        testing.go:1591: panic: runtime error: index out of range [-1]
            goroutine 4015806 [running]:
            runtime/debug.Stack()
            	/opt/homebrew/Cellar/go/1.23.1/libexec/src/runtime/debug/stack.go:26 +0xc4
            testing.tRunner.func1()
            	/opt/homebrew/Cellar/go/1.23.1/libexec/src/testing/testing.go:1591 +0x21c
            panic({0x1030bc1a0?, 0x1400a1521f8?})
            	/opt/homebrew/Cellar/go/1.23.1/libexec/src/runtime/panic.go:785 +0x124
            github.com/Eyevinn/mp4ff/mp4.DecodeMimeSR({{0x1400a68e9e0?, 0x1400a6a3f20?}, 0x1400a68e9e0?, 0x8?}, 0x14009e8c658?, {0x1030df818, 0x1400a68cd40})
            	/Users/eric/src/mp4ff/mp4/mime.go:35 +0x214
            github.com/Eyevinn/mp4ff/mp4.DecodeMime({{0x1400a68e9e0?, 0x140001207b0?}, 0x1400a68e9e0?, 0x4?}, 0x0, {0x1030d8da0?, 0x1400a6a3f20?})
            	/Users/eric/src/mp4ff/mp4/mime.go:24 +0x184
            github.com/Eyevinn/mp4ff/mp4.DecodeBox(0x0, {0x1030d8da0, 0x1400a6a3f20})
            	/Users/eric/src/mp4ff/mp4/box.go:315 +0x178
            github.com/Eyevinn/mp4ff/mp4.FuzzDecodeBox.func1(0x1400a6c29c0, {0x1400a112d80, 0xc, 0x10})
            	/Users/eric/src/mp4ff/mp4/fuzz_test.go:65 +0x14c

[tobbee]

This is fine, except the description of the size.