[ContextManager] Re-initialze contexts if necessary

In case a Context has been retrieved from the MemoryManager, without
it malloced, then we need to re-initialze some of the Context's values
before we can use the Context.

emper/Context.hpp

@@ -106,6 +106,14 @@ class EMPER_CONTEXT_ALIGNAS Context : Logger<LogSubsystem::C> {
 	static auto calcBos(char* context) -> void*;
 	auto calcBos() -> void*;
 
+	/*
+	 * Used by ContextManager to re-initialize a cached unused context.
+	 */
+	inline void reinit() {
+		currentFiber = nullptr;
+		savedStackpointer = const_cast<void*>(alphaFunctionIpLocation);
+	}
+
  public:
 	Context(func_t mainFunction);
 

emper/ContextManager.cpp

@@ -31,6 +31,7 @@ auto ContextManager::getFreeContext() -> Context* {
 	}
 
 	auto* context = static_cast<Context*>(memory);
+	context->reinit();
 	if constexpr (emper::STATS_STACK_USAGE == emper::StatsStackUsage::accurate) {
 		// If we want accurate stack-usage stats, then we need to re-mark
 		// the stack if we got the memory from the memory manager but if