Fix assertion failure when failing to detect encoding

Looks like prior to 7.3 this just passed the original string
through. Since 7.3 it returns false. Let's stick with that
behavior.

Author: nikic

ext/mbstring/mbstring.c

@@ -2489,9 +2489,6 @@ MBSTRING_API char *php_mb_convert_encoding(const char *input, size_t length, con
 	if (output_len) {
 		*output_len = 0;
 	}
-	if (!input) {
-		return NULL;
-	}
 
 	/* pre-conversion encoding */
 	ZEND_ASSERT(num_from_encodings >= 1);
@@ -2507,7 +2504,7 @@ MBSTRING_API char *php_mb_convert_encoding(const char *input, size_t length, con
 			&string, from_encodings, num_from_encodings, MBSTRG(strict_detection));
 		if (!from_encoding) {
 			php_error_docref(NULL, E_WARNING, "Unable to detect character encoding");
-			from_encoding = &mbfl_encoding_pass;
+			return NULL;
 		}
 	}
 

ext/mbstring/tests/mb_convert_encoding_failed_detection.phpt

@@ -0,0 +1,11 @@
+--TEST--
+mb_convert_encoding() when encoding detection fails
+--FILE--
+<?php
+
+var_dump(mb_convert_encoding("\xff", "ASCII", ["UTF-8", "UTF-16"]));
+
+?>
+--EXPECTF--
+Warning: mb_convert_encoding(): Unable to detect character encoding in %s on line %d
+bool(false)