Nil pointer dereference panic in `Find()` if module by prefix is not found

[sengleung]

Panic stack trace of goyang source files:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x15a7cc0]

goroutine 20 [running]:
panic({0x18493c0, 0x1fe79a0})
        /usr/local/go/src/runtime/panic.go:884 +0x213 fp=0xc003b1ecd8 sp=0xc003b1ec18 pc=0x1036193
runtime.panicmem(...)
        /usr/local/go/src/runtime/panic.go:260
runtime.sigpanic()
        /usr/local/go/src/runtime/signal_unix.go:841 +0x37d fp=0xc003b1ed38 sp=0xc003b1ecd8 pc=0x104db1d
github.com/openconfig/goyang/pkg/yang.(*Module).ParentNode(0x159ea5f?)
        github.com/openconfig/goyang/pkg/yang/yang.go:143 fp=0xc003b1ed40 sp=0xc003b1ed38 pc=0x15a7cc0
github.com/openconfig/goyang/pkg/yang.RootNode({0x1af0840?, 0x0?})
        github.com/openconfig/goyang/pkg/yang/node.go:147 +0x45 fp=0xc003b1ed68 sp=0xc003b1ed40 pc=0x159f0e5
github.com/openconfig/goyang/pkg/yang.module({0x1af0840?, 0x0?})
        github.com/openconfig/goyang/pkg/yang/node.go:159 +0x27 fp=0xc003b1ed98 sp=0xc003b1ed68 pc=0x159f167
github.com/openconfig/goyang/pkg/yang.(*Entry).Find(0xc00660cf00, {0xc001625f80?, 0xc003b1ef40?})
        github.com/openconfig/goyang/pkg/yang/entry.go:1325 +0x425 fp=0xc003b1ee80 sp=0xc003b1ed98 pc=0x1595c85

https://github.com/openconfig/goyang/blob/944052f4c47385554f2fb0f46ca4ecfbf446450c/pkg/yang/entry.go#L1325

• Find() calls FindModuleByPrefix() which returns nil of type *yang.Module.
• module() is called with (*yang.Module)(nil).

https://github.com/openconfig/goyang/blob/944052f4c47385554f2fb0f46ca4ecfbf446450c/pkg/yang/node.go#L158-L160

https://github.com/openconfig/goyang/blob/944052f4c47385554f2fb0f46ca4ecfbf446450c/pkg/yang/node.go#L146-L147

https://github.com/openconfig/goyang/blob/944052f4c47385554f2fb0f46ca4ecfbf446450c/pkg/yang/yang.go#L142

• module() calls RootNode() which expects the Node interface type. (*yang.Module)(nil) is passed into RootNode().
• ParentNode() is called with (*yang.Module)(nil). This is not n == nil because the interface has a nil pointer value (same explanation as this). This results in a nil pointer dereference panic.
• RootNode() is vulnerable to panics due to a non-nil interface (e.g., when a nil *yang.Module is passed). Even if we properly nil check in RootNode() with reflection, we will again be vulnerable to a panic again with Kind() in node.go:160.

The root cause here is that module() is called with (*yang.Module)(nil). We should nil check FindModuleByPrefix() before calling module() to prevent the panic.