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/unicode_strings.i

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