Check newly created consistency using _PyUnicode_CheckConsistency(str, 1)

* In debug mode, fill the string data with invalid characters
 * Simplify also reference counting in PyCodec_BackslashReplaceErrors()
   and PyCodec_XMLCharRefReplaceError()

Modules/_json.c

@@ -246,6 +246,7 @@ ascii_escape_unicode(PyObject *pystr)
         }
     }
     output[chars++] = '"';
+    assert(_PyUnicode_CheckConsistency(rval, 1));
     return rval;
 }
 

Modules/md5module.c

@@ -397,6 +397,7 @@ MD5_hexdigest(MD5object *self, PyObject *unused)
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }
 

Modules/sha1module.c

@@ -373,6 +373,7 @@ SHA1_hexdigest(SHA1object *self, PyObject *unused)
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }
 

Modules/sha256module.c

@@ -466,6 +466,7 @@ SHA256_hexdigest(SHAobject *self, PyObject *unused)
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }
 

Modules/sha512module.c

@@ -532,6 +532,7 @@ SHA512_hexdigest(SHAobject *self, PyObject *unused)
         c = (digest[i] & 0xf);
         hex_digest[j++] = Py_hexdigits[c];
     }
+    assert(_PyUnicode_CheckConsistency(retval, 1));
     return retval;
 }