Fix MarkGUCPrefixReserved() to check all options.

This bug was only present on v15. MarkGUCPrefixReserved() is new in
v15, and in v16, it was rewritten to use a hash table and the new
implementation did not have this bug.

Author: Karina Litskevich, Ekaterina Sokolova
Discussion: https://www.postgresql.org/message-id/CACiT8ibqyC=_g1n6FXyFJvFW0BEjAH3_5aGqUSFeEp8GpnVrhw@mail.gmail.com

src/backend/utils/misc/guc.c

@@ -9723,6 +9723,7 @@ MarkGUCPrefixReserved(const char *className)
 			num_guc_variables--;
 			memmove(&guc_variables[i], &guc_variables[i + 1],
 					(num_guc_variables - i) * sizeof(struct config_generic *));
+			i--;
 		}
 	}