Add missing checks for failures in calls to PyUnicode_AsUTF8String.

Previously a seg fault could occur when passing invalid UTF8 strings (low
surrogates), eg passing u"\udcff" to the C layer (Python 3).

Examples/test-suite/python/unicode_strings_runme.py

@@ -25,3 +25,13 @@ if sys.version_info[0:2] < (3, 0):
     check(unicode_strings.charstring(unicode("hello4")), "hello4")
     unicode_strings.charstring(u"hell\xb05")
     unicode_strings.charstring(u"hell\u00f66")
+
+low_surrogate_string = u"\udcff"
+try:
+    unicode_strings.instring(low_surrogate_string)
+    # Will succeed with Python 2
+except TypeError, e:
+    # Python 3 will fail the PyUnicode_AsUTF8String conversion resulting in a TypeError.
+    # The real error is actually:
+    # UnicodeEncodeError: 'utf-8' codec can't encode character '\udcff' in position 0: surrogates not allowed
+    pass

Examples/test-suite/python_varargs_typemap.i

@@ -23,8 +23,11 @@
        SWIG_fail;
     }
     pystr = PyUnicode_AsUTF8String(pyobj);
+    if (!pystr) {
+      SWIG_fail;
+    }
     str = strdup(PyBytes_AsString(pystr));
-    Py_XDECREF(pystr);
+    Py_DECREF(pystr);
 %#else  
     if (!PyString_Check(pyobj)) {
        PyErr_SetString(PyExc_ValueError, "Expected a string");

Examples/test-suite/unicode_strings.i

@@ -20,4 +20,6 @@ char *charstring(char *s) {
   return s;
 }
 
+void instring(const char *s) {
+}
 %}