PHPC-615: Return after throwing for bson_reader_read() error

This adds additional error tests for toJSON(), which demonstrate the problem when both exceptions might have been thrown.

Additionally, this ensures that the bson_reader_t is freed before returning. Previously, the code relied on the free after the second exception was thrown.

src/bson.c

@@ -1587,6 +1587,8 @@ PHP_FUNCTION(toJSON)
 		bson_free(str);
 	} else {
 		phongo_throw_exception(PHONGO_ERROR_UNEXPECTED_VALUE TSRMLS_CC, "Could not read document from BSON reader");
+		bson_reader_destroy(reader);
+		return;
 	}
 
 	if (bson_reader_read(reader, &eof) || !eof) {

tests/bson/bson-toJSON_error-002.phpt

@@ -0,0 +1,28 @@
+--TEST--
+BSON\toJSON(): BSON decoding exceptions for malformed documents
+--SKIPIF--
+<?php require __DIR__ . "/../utils/basic-skipif.inc"?>
+--FILE--
+<?php
+require_once __DIR__ . "/../utils/basic.inc";
+
+$tests = array(
+    pack('Vx', 4), // Empty document with invalid length (too small)
+    pack('Vx', 6), // Empty document with invalid length (too large)
+);
+
+foreach ($tests as $bson) {
+    echo throws(function() use ($bson) {
+        toJSON($bson);
+    }, 'MongoDB\Driver\Exception\UnexpectedValueException'), "\n";
+}
+
+?>
+===DONE===
+<?php exit(0); ?>
+--EXPECTF--
+OK: Got MongoDB\Driver\Exception\UnexpectedValueException
+Could not read document from BSON reader
+OK: Got MongoDB\Driver\Exception\UnexpectedValueException
+Could not read document from BSON reader
+===DONE===