Return false in Reader::readValue when stack limit is exceeded

jsoncpp, as a shared library, should not call `abort` merely because there's an error reading a value.

See https://en.cppreference.com/w/c/program/abort, `abort` should only be called to **abnormally** cause the program to exit. Functions inserted by `atexit` are also not called, meaning that the host program may have not cleaned up resources properly. But here, exceeding stack limit isn't a sign of abnormalty.

`exit` is not a good substitute either, see the `exit-in-shared-library` from Debian:
https://lintian.debian.org/tags/exit-in-shared-library.html

Fix #1618

In this case, returning false seems like a better idea.

src/lib_json/json_reader.cpp

@@ -143,7 +143,12 @@ bool Reader::readValue() {
   // after calling readValue(). parse() executes one nodes_.push(), so > instead
   // of >=.
   if (nodes_.size() > stackLimit_g)
+#if JSON_USE_EXCEPTION
     throwRuntimeError("Exceeded stackLimit in readValue().");
+#else
+    // throwRuntimeError aborts. Don't abort here.
+    return false;
+#endif
 
   Token token;
   readTokenSkippingComments(token);