Fix endless loop on srp app when listing users

With the -list option the srp app loops on the main while() endlessly,
whether users were given on the command line or not. The loop should
be stopped when in list mode and there are no more users.
Reviewed-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/3489)

apps/srp.c

@@ -37,13 +37,14 @@ static int get_index(CA_DB *db, char *id, char type)
     int i;
     if (id == NULL)
         return -1;
-    if (type == DB_SRP_INDEX)
+    if (type == DB_SRP_INDEX) {
         for (i = 0; i < sk_OPENSSL_PSTRING_num(db->db->data); i++) {
             pp = sk_OPENSSL_PSTRING_value(db->db->data, i);
             if (pp[DB_srptype][0] == DB_SRP_INDEX
                 && strcmp(id, pp[DB_srpid]) == 0)
                 return i;
-    } else
+        }
+    } else {
         for (i = 0; i < sk_OPENSSL_PSTRING_num(db->db->data); i++) {
             pp = sk_OPENSSL_PSTRING_value(db->db->data, i);
 
@@ -51,6 +52,7 @@ static int get_index(CA_DB *db, char *id, char type)
                 && strcmp(id, pp[DB_srpid]) == 0)
                 return i;
         }
+    }
 
     return -1;
 }
@@ -549,10 +551,11 @@ int srp_main(int argc, char **argv)
                 doupdatedb = 1;
             }
         }
-        if (--argc > 0)
+        if (--argc > 0) {
             user = *(argv++);
-        else {
-            user = NULL;
+        } else {
+            /* no more processing in any mode if no users left */
+            break;
         }
     }