os: add invalid path exception to Path_base

In case the path is invalid during import raise the 'Path_invalid'
exception.

Issue #4088

repos/os/include/os/path.h

@@ -30,6 +30,7 @@ class Genode::Path_base
 	public:
 
 		class Path_too_long { };
+		class Path_invalid  { };
 
 	protected:
 
@@ -196,6 +197,9 @@ class Genode::Path_base
 
 		void import(char const *path, char const *pwd = 0)
 		{
+			if (!path)
+				throw Path_invalid();
+
 			/*
 			 * Validate 'pwd' argument, if not supplied, enforce invariant
 			 * that 'pwd' is an absolute path.