mpegts: test for allocation failure

While it's unlikely to get there, it silences the coverity warning
on the error code path that we test for NULL before freeing, when
all branches there are from locations where pmt cannot be NULL,
and removing the NULL check makes the code more vulnerable to a
hypothetical future branch from somewhere where it can be.

Coverity 1139852

gst-libs/gst/mpegts/gstmpegtssection.c

@@ -657,6 +657,10 @@ _parse_pmt (GstMpegTsSection * section)
   guint stream_info_length;
 
   pmt = g_slice_new0 (GstMpegTsPMT);
+  if (!pmt) {
+    GST_WARNING ("Failed to allocate PMT structure");
+    goto error;
+  }
 
   data = section->data;
   end = data + section->section_length;