Tools: Test: In src_test.m prevent error with empty plot handle

Octave errors if plot handle is empty, Matlab doesn't. This can
happen if frequency response test data read fails. In that case
an empty plot window is stored as indication of error.

Signed-off-by: Seppo Ingalsuo <seppo.ingalsuo@linux.intel.com>

Author: singalsu

tools/test/audio/src_test.m

@@ -465,7 +465,7 @@
 
 function src_test_result_print(t, testverbose, testacronym, ph)
 tstr = sprintf('%s SRC %d, %d', testverbose, t.fs1, t.fs2);
-if nargin > 3
+if nargin > 3 && ~isempty(ph)
         title(ph, tstr);
 else
         title(tstr);