path-util: NULL strings are definitely not valid paths

Let's make this functions that check validity of paths a bit more
friendly towards one specific kind of invalid path: a NULL pointer.

This follows similar logic in path_is_valid(), path_is_normalized() and
so on.

src/basic/path-util.h

@@ -43,12 +43,16 @@
 #endif
 
 static inline bool is_path(const char *p) {
-        assert(p);
+        if (!p) /* A NULL pointer is definitely not a path */
+                return false;
+
         return strchr(p, '/');
 }
 
 static inline bool path_is_absolute(const char *p) {
-        assert(p);
+        if (!p) /* A NULL pointer is definitely not an absolute path */
+                return false;
+
         return p[0] == '/';
 }