Fix crashes in FCGI_PARAMS parsing

.gitignore

@@ -3,3 +3,4 @@ example_clock
 example_simple
 fastcgi_conn_afl
 *.o
+afl_state/findings

afl_state/findings/.cur_input

@@ -1 +0,0 @@
-<EFBFBD>

afl_state/testcases/5d86ac01a848cb9448a52eb53c21516982a56b70222a4d96b130171228bcf37c

@@ -0,0 +1 @@
+
9))))

buffer.cc

@@ -9,7 +9,9 @@ size_t ConstBuffer::ReadMaxLen() const {
 }
 
 const char *ConstBuffer::Read(size_t len) {
-	CHECK_LE(len, ReadMaxLen());
+	if (ReadMaxLen() < len) {
+		return nullptr;
+	}
 	const auto *ret = &const_buf_[start_];
 	start_ += len;
 	return ret;

fastcgi_conn.cc

@@ -82,8 +82,25 @@ int FastCGIConn::Read() {
 				ConstBuffer param_buf(buf_.Read(header->ContentLength()), header->ContentLength());
 				while (param_buf.ReadMaxLen() > 0) {
 					const auto *param_header = param_buf.ReadObj<FastCGIParamHeader>();
-					std::string_view key(param_buf.Read(param_header->key_length), param_header->key_length);
-					std::string_view value(param_buf.Read(param_header->value_length), param_header->value_length);
+					if (!param_header) {
+						LOG(ERROR) << "FCGI_PARAMS missing header";
+						return sock_;
+					}
+
+					const auto *key_buf = param_buf.Read(param_header->key_length);
+					if (!key_buf) {
+						LOG(ERROR) << "FCGI_PARAMS missing key";
+						return sock_;
+					}
+					std::string_view key(key_buf, param_header->key_length);
+
+					const auto *value_buf = param_buf.Read(param_header->value_length);
+					if (!value_buf) {
+						LOG(ERROR) << "FCGI_PARAMS missing value";
+						return sock_;
+					}
+					std::string_view value(value_buf, param_header->value_length);
+
 					if (headers_.find(key) != headers_.end()) {
 						request_->AddParam(key, value);
 					}