broadband-modem-mbim: initialize autofreed string to NULL

device_caps_query_ready() declares caps_custom_data_class_str as
g_autofree without an initializer. On every early error return
(command failure such as a transaction timeout or NotOpened, or a
response parse failure) the variable is never assigned, so the
autofree cleanup runs g_free() on uninitialized stack garbage,
crashing ModemManager with a glibc "invalid free" abort (or SIGSEGV)
whenever the device caps query fails during modem initialization.

Fixes: 19c817cfe9 ("broadband-modem-mbim: consistently handle custom data classes")
Closes: #1094

Signed-off-by: Semyon Maryasin <simeon@maryasin.name>

src/mm-broadband-modem-mbim.c

@@ -519,7 +519,7 @@ device_caps_query_ready (MbimDevice   *device,
     GError                         *error = NULL;
     LoadCurrentCapabilitiesContext *ctx;
     MbimDataClass                   caps_data_class = MBIM_DATA_CLASS_NONE;
-    g_autofree gchar               *caps_custom_data_class_str;
+    g_autofree gchar               *caps_custom_data_class_str = NULL;
 
     self = g_task_get_source_object (task);
     ctx  = g_task_get_task_data (task);