Segfaults con todas las declaraciones `if/else-if/else-if` pero no al comentar una de ellas

I have a tricky one here I just can't seem to figure out. When using the whole if/else-if/else-if it segfaults if the argument in the vector doesn't exist. If the argument exists then it runs fine it seems. When I comment out one of the else-if blocks then it runs ok it seems also.

Editado:

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; i++)
    {
        if ((i == argcp -1) && (strncmp (argvp[i], "-P", 2) != 0))
        {
            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;
        }
        else 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;
        }
    }     
    file_input (argvp, argcp);
}

Y aquí está la salida de g_print () dentro de for () loop with line 56 demostración argvp y line 57 demostración argcp :

From `rt_priority.c` line 42: 6
From `rt_priority.c` line 56: /usr/bin/jackd
From `rt_priority.c` line 57: 6
From `rt_priority.c` line 56: -dalsa
From `rt_priority.c` line 57: 6
From `rt_priority.c` line 56: -dhw:M2496
From `rt_priority.c` line 57: 6
From `rt_priority.c` line 56: -r48000
From `rt_priority.c` line 57: 6
From `rt_priority.c` line 56: -p128
From `rt_priority.c` line 57: 6
From `rt_priority.c` line 56: -n2
From `rt_priority.c` line 57: 6
From `rt_priority.c` line 56: (null)
From `rt_priority.c` line 57: 6
Segmentation fault (core dumped)

Editado:

Este es el get_arg_vector () función:

gchar **argvp
get_arg_vector ()
{
    gchar cmd[128];
gchar **argvp;
gchar *contents;
gint argcp;
gsize size;

/* Create path to file `.jackdrc` using `g_sprintf ()`. */
g_sprintf (cmd, "%s/.jackdrc", g_getenv ("HOME"));

/* Check if file path exists. */
if (g_file_test (cmd, G_FILE_TEST_EXISTS) == FALSE)
{
    g_print ("File doesn't exist.  Create file.");

    return NULL;
}

g_file_get_contents (cmd, &contents, &size, NULL);
g_shell_parse_argv (contents, &argcp, &argvp, NULL);

return argvp;
}