Check for integer overflow in cppbor::parseRecursively.

Bug: 191303307
Test: Fuzzer from bug
Change-Id: I98830ec1bd77f152266f35a585d286be13a2551b

src/cppbor_parse.cpp

diff --git a/src/cppbor_parse.cpp b/src/cppbor_parse.cpp
index f5e8fcf..964a72d 100644
--- a/src/cppbor_parse.cpp
+++ b/src/cppbor_parse.cpp
@@ -96,7 +96,8 @@
                                                       const uint8_t* valueBegin, const uint8_t* end,
                                                       const std::string& errLabel,
                                                       ParseClient* parseClient) {
-    if (end - valueBegin < static_cast<ssize_t>(length)) {
+    ssize_t signed_length = static_cast<ssize_t>(length);
+    if (end - valueBegin < signed_length || signed_length < 0) {
         parseClient->error(hdrBegin, insufficientLengthString(length, end - valueBegin, errLabel));
         return {hdrBegin, nullptr /* end parsing */};
     }