Don't fall back to vtbl_pass if no matching vtbl found

If we don't know how to convert between two encodings, make sure
we error instead of ignoring the issue.

Explicitly use vtbl_pass if we are round-tripping wchar->wchar or
8bit->8bit. Fingers crossed that nothing else relies on the
vtbl_pass fallback...

Author: nikic

ext/mbstring/libmbfl/mbfl/mbfilter.c

@@ -147,7 +147,7 @@ mbfl_buffer_converter_new(
 
 	/* create convert filter */
 	convd->filter1 = NULL;
-convd->filter2 = NULL;
+	convd->filter2 = NULL;
 	if (mbfl_convert_filter_get_vtbl(convd->from, convd->to) != NULL) {
 		convd->filter1 = mbfl_convert_filter_new(convd->from, convd->to, mbfl_memory_device_output, NULL, &convd->device);
 	} else {
@@ -164,6 +164,7 @@ convd->filter2 = NULL;
 		}
 	}
 	if (convd->filter1 == NULL) {
+		mbfl_free(convd);
 		return NULL;
 	}
 
@@ -2399,7 +2400,7 @@ mime_header_decoder_new(const mbfl_encoding *outcode)
 	mbfl_memory_device_init(&pd->tmpdev, 0, 0);
 	pd->cspos = 0;
 	pd->status = 0;
-	pd->encoding = &mbfl_encoding_pass;
+	pd->encoding = &mbfl_encoding_8bit;
 	pd->incode = &mbfl_encoding_ascii;
 	pd->outcode = outcode;
 	/* charset convert filter */

ext/mbstring/libmbfl/mbfl/mbfl_convert.c

@@ -171,9 +171,8 @@ mbfl_convert_filter_new(
 	const struct mbfl_convert_vtbl *vtbl;
 
 	vtbl = mbfl_convert_filter_get_vtbl(from, to);
-
 	if (vtbl == NULL) {
-		vtbl = &vtbl_pass;
+		return NULL;
 	}
 
 	/* allocate */
@@ -454,6 +453,10 @@ const struct mbfl_convert_vtbl * mbfl_convert_filter_get_vtbl(
 		to = &mbfl_encoding_8bit;
 	}
 
+	if (to == from && (to == &mbfl_encoding_wchar || to == &mbfl_encoding_8bit)) {
+		return &vtbl_pass;
+	}
+
 	if (to->no_encoding == mbfl_no_encoding_wchar) {
 		return from->input_filter;
 	} else if (from->no_encoding == mbfl_no_encoding_wchar) {