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mozilla: handle wpa_bss_get_ssid() returning NULL

Will Thompson requested to merge wjt/geoclue:null-ssid-gvariant into master

On my Endless OS system running 2.5.2, I see the following crash reasonably often:

    #0  0x00007fdc12e57300 in g_bit_lock () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
    #1  0x00007fdc12ec38f7 in g_variant_n_children () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
    #2  0x00005562e11de273 in variant_to_string (variant=0x0, len=len@entry=0x0) at ../src/gclue-mozilla.c:49
            n_bytes = <optimized out>
            i = <optimized out>
            ret = <optimized out>
    #3  0x00005562e11de4f8 in get_ssid_from_bss (bss=0x5562e2c896d0) at ../src/gclue-mozilla.c:71
            variant = <optimized out>
            variant = <optimized out>
    #4  gclue_mozilla_should_ignore_bss (bss=bss@entry=0x5562e2c896d0) at ../src/gclue-mozilla.c:414
            ssid = <optimized out>
            bssid = <optimized out>
    #5  0x00005562e11ddaba in on_bss_proxy_ready (source_object=<optimized out>, res=<optimized out>, user_data=0x5562e2c7a2d0) at ../src/gclue-wifi.c:313
            wifi = 0x5562e2c7a2d0
            bss = 0x5562e2c896d0
            error = 0x0
            ssid = <optimized out>

In frame 2, 'variant' is NULL. In 2.5.2, get_ssid_from_bss looks like this:

static gboolean
get_ssid_from_bss (WPABSS *bss)
{
        GVariant *variant = wpa_bss_get_ssid (bss);

        return variant_to_string (variant, NULL);
}

Since 2.5.2, bdaf6069 added a NULL check to this function – which I think would fix my crash. Then 3b7a7d2a removed this NULL check again without explanation!

It seems that, in practice, wpa_bss_get_ssid() can return NULL, so we must handle this case.

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