parsers: fix leak of err when asciilower hits a unicode decode error

This is one of many errors detected in parsers.c by cpychecker[1]. I
haven't gone through all of them yet.

1: https://gcc-python-plugin.readthedocs.org/en/latest/index.html

mercurial/parsers.c

@@ -115,6 +115,7 b' static PyObject *asciilower(PyObject *se'
 				"ascii", str, len, i, (i + 1),
 				"unexpected code byte");
 			PyErr_SetObject(PyExc_UnicodeDecodeError, err);
+			Py_XDECREF(err);
 			goto quit;
 		}
 		newstr[i] = lowertable[(unsigned char)c];