Segfaults with the whole `if/else-if/else-if' statement

void
priority_arg_create (gchar *arg)
{
	gchar **argvp;
	gint argcp;
	gint i;
	gint j;

	argvp = get_arg_vector ();
	argcp = 0;
	i = 0;

	/* obtain arg count not including the `NULL` terminator. */
	while (argvp[argcp])
	{
		argcp++;
	}

	g_print ("From `rt_priority.c` line 42: %d\n", argcp);

	/* Here we look for the jack arg `-Px` with the for loop then the if
	statement. If it does not exist then we add it to arg vector with
	`else/if` statement. */
	for (i = 0; i <= argcp + 1; i++)
	{
		if (g_strcmp0 (argvp[i], arg) == 0)
		{
			g_print ("From `rt_priority.c` line 51: %d\n", i);

			break;
		}
		/* If `priority arg` is found but the number does not match
		execute else/if statement. */
		else if (strncmp (argvp[i], "-P", 2) == 0)
		{
			argvp[i] = arg;

			break;
		}
		else if (argvp[i] == NULL)
		{
			g_print ("From `rt_priority.c` line 65\n");

			/* Add space to arg vector for the jackd arg `-R`. */
			argcp = argcp + 1;

			/* If realtime arg exists then place priority arg right
			after that.*/
			if ((strncmp (argvp[1], "-r", 2) == 0) ||
				(strncmp (argvp[1], "-R", 2) == 0))
			{
				/* Here we move the args over one to place `-Px` as the
				third arg in the vector. */
				for (j = argcp; j >= 2; j--)
				{
					argvp[j] = argvp[j - 1];
				}

				argvp[2] = arg;
			}
			else
			{
				/* Here we move the args over one to place `-Px` as the
				second arg in the vector. */
				for (j = argcp; j >= 1; j--)
				{
					argvp[j] = argvp[j - 1];
				}

				argvp[1] = arg;
			}

			break;
		}
	}

	file_input (argvp, argcp);
}