fix(callstack): guard against NULL from strchr when parsing symbols

[Rakdos8]

Problem

In the POSIX/macOS backtrace_symbols path, CCPCallstack::Enumerate parses each symbol line with const char* plus = strchr( lines[i], '+' ); if( *plus ). When a line has no + (stripped binaries, static or symbol-less frames),
strchr returns NULL and *plus dereferences it, crashing while dumping a callstack. This path is used by macOS
(it is neither _MSC_VER nor __ANDROID__), so it is a real crash risk on a primary target — triggered when
capturing and dumping callstacks (e.g. memory-tracker reports with callstack capture enabled).

Fix

Check the pointer before dereferencing: if( plus && *plus ). One-line defensive change; lines without + are simply
treated as having no demangle-able suffix, same as the existing fallback.

Tests

No dedicated test: the parser consumes backtrace_symbols output directly and is not injectable from a unit test, so
the no-+ branch cannot be exercised deterministically without extracting the parser into a testable function (out of
scope for a one-line fix). The existing CanDumpSymbolsIntoFile and DepthIsEqualToItemCount tests cover the
nominal path and guard against regressions there. Reviewed manually; not compiled locally as the vcpkg toolchain is
unavailable in this environment.

Scope

POSIX/macOS path only. The Windows (_MSC_VER) and Android paths are unchanged. No public API change.