Fix -Werror=maybe-uninitialized warning

GCC with AddressSanitizer reports that pathp in map_insert() may be
used uninitialized:

src/map.c:593:15: error: pathp may be used uninitialized
  593 |     for (pathp--; (uintptr_t) pathp >= (uintptr_t) path; pathp--)

While the program logic ensures pathp is always initialized through
rb_insert_unique() before use, static analysis cannot prove this due
to the pointer-to-pointer assignment pattern. Initialize to NULL to
satisfy the compiler and improve defensive programming.

Author: jserv

src/map.c

@@ -572,7 +572,7 @@ bool map_insert(map_t obj, const void *key, const void *val)
         return false;
 
     rb_path_entry_t path[RB_MAX_DEPTH];
-    rb_path_entry_t *pathp;
+    rb_path_entry_t *pathp = NULL;
 
     /* Single traversal to check existence and get insertion point */
     const map_node_t *existing = rb_insert_unique(obj, key, path, &pathp);