core: better handle NULL spew from serial ports during probing

Observed on a generic ZTE device.

src/mm-plugin-base.c

@@ -515,6 +515,11 @@ static const char *dq_strings[] = {
     NULL
 };
 
+static guint8 zerobuf[32] = {
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+    0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0
+};
+
 static void
 probed_info_free (MMPluginBaseProbedInfo *info)
 {
@@ -535,6 +540,13 @@ port_buffer_full (MMSerialPort *port, GByteArray *buffer, gpointer user_data)
     size_t iter_len;
     int i;
 
+    /* Some devices (observed on a ZTE branded "QUALCOMM INCORPORATED" model
+     * "154") spew NULLs from some ports.
+     */
+    if (   (buffer->len >= sizeof (zerobuf))
+        && (memcmp (buffer->data, zerobuf, sizeof (zerobuf)) == 0))
+        goto stop_probing;
+
     /* Check for an immediate disqualification response.  There are some
      * ports (Option Icera-based chipsets have them, as do Qualcomm Gobi
      * devices before their firmware is loaded) that just shouldn't be
@@ -551,15 +563,18 @@ port_buffer_full (MMSerialPort *port, GByteArray *buffer, gpointer user_data)
         for (i = 0; i < buffer->len - iter_len; i++) {
             if (!memcmp (&buffer->data[i], *iter, iter_len)) {
                 /* Immediately close the port and complete probing */
+                goto stop_probing;
+            }
+        }
+    }
+    return;
+
+stop_probing:
     priv->probed_caps = 0;
     priv->probed_vendor = NULL;
     priv->probed_product = NULL;
     mm_serial_port_close (MM_SERIAL_PORT (priv->probe_port));
     probe_complete (task);
-                return;
-            }
-        }
-    }
 }
 
 static gboolean