Fix segfaults caused by py_unicode_to_char_star

PyString_AsString returns a pointer to the PyString's buffer. So when we
Py_DECREF the string immediately afterwards, this buffer is already free'd.
It still works, but is not reliable.

Instead, move more of the processing to Perl 6, so we can safely py_dec_ref
the string after NativeCall copied it to a Perl 6 string.

Fixes htmlify.p6 in perl6/doc.git

lib/Inline/Python.pm6

@@ -105,9 +105,12 @@ sub py_float_as_double(OpaquePointer)
 sub py_float_to_py(num64)
     returns OpaquePointer { ... }
     native(&py_float_to_py);
-sub py_unicode_to_char_star(OpaquePointer)
+sub py_unicode_as_utf8_string(OpaquePointer)
+    returns OpaquePointer { ... }
+    native(&py_unicode_as_utf8_string);
+sub py_string_as_string(OpaquePointer)
     returns Str { ... }
-    native(&py_unicode_to_char_star);
+    native(&py_string_as_string);
 sub py_string_to_buf(OpaquePointer, CArray[CArray[int8]])
     returns Int { ... }
     native(&py_string_to_buf);
@@ -226,7 +229,10 @@ method py_to_p6(OpaquePointer $value) {
         return py_float_as_double($value);
     }
     elsif py_unicode_check($value) {
-        return py_unicode_to_char_star($value);
+        my $string = py_unicode_as_utf8_string($value) or return;
+        my $p6_str = py_string_as_string($string);
+        py_dec_ref($string);
+        return $p6_str;
     }
     elsif py_string_check($value) {
         my $string_ptr = CArray[CArray[int8]].new;

pyhelper.c

@@ -118,24 +118,17 @@ PyObject *py_buf_to_py(int len, char *buf) {
     return PyString_FromStringAndSize(buf, len);
 }
 
-char *py_unicode_to_char_star(PyObject *obj) {
-    PyObject * const string = PyUnicode_AsUTF8String(obj);    /* new reference */
-    if (!string) {
-        return NULL;
-    }
-    char * const str = PyString_AsString(string);
-    Py_DECREF(string);
-    return str;
+PyObject *py_unicode_as_utf8_string(PyObject *obj) {
+    return PyUnicode_AsUTF8String(obj);    /* new reference */
+}
+
+char *py_string_as_string(PyObject *obj) {
+    return PyString_AsString(obj);
 }
 
 Py_ssize_t py_string_to_buf(PyObject *obj, char **buf) {
-    PyObject * const string = PyObject_Str(obj);    /* new reference */
-    if (!string) {
-        return 0;
-    }
     Py_ssize_t length;
     PyString_AsStringAndSize(obj, buf, &length);
-    Py_DECREF(string);
     return length;
 }