clipmenu: Please -fexceptions

When compiling with -fexceptions (only) for some reason, the following
can be seen:

    src/clipmenu.c: In function ‘main’:
    src/clipmenu.c:183:9: error: ‘hash’ may be used uninitialized [-Werror=maybe-uninitialized]
    183 |         run_clipserve(hash);
	|         ^~~~~~~~~~~~~~~~~~~
    src/clipmenu.c:179:14: note: ‘hash’ was declared here
    179 |     uint64_t hash;
	|              ^~~~

...but this seems off, we always initialise it if dmenu_exit_code is
EXIT_SUCCESS, and no other static analyser has a problem with it.
Anyway, this is enough.

src/clipmenu.c

@@ -16,6 +16,8 @@
 static int dmenu_user_argc;
 static char **dmenu_user_argv;
 
+#define HASH_INVALID UINT64_MAX
+
 /**
  * Calculate the base 10 padding length for a number.
  */
@@ -136,6 +138,7 @@ static int _nonnull_ interact_with_dmenu(struct config *cfg, int *input_pipe,
     int forced_ret = 0;
     if (str_to_uint64(sel_idx_str, &sel_idx) < 0 || sel_idx == 0 ||
         sel_idx > cur_clips) {
+        *out_hash = HASH_INVALID;
         forced_ret = EXIT_FAILURE;
     } else {
         *out_hash = idx_to_hash[sel_idx - 1];
@@ -179,7 +182,7 @@ int main(int argc, char *argv[]) {
     uint64_t hash;
     int dmenu_exit_code = prompt_user_for_hash(&cfg, &hash);
 
-    if (dmenu_exit_code == EXIT_SUCCESS) {
+    if (dmenu_exit_code == EXIT_SUCCESS && hash != HASH_INVALID) {
         run_clipserve(hash);
     }