Rework defining variables on command-line

Avoids allocating memory
Detects CLI errors even when failing to open file
Reports symbols as defined on command-line instead of input line 0 (!?)

src/asm/fstack.c

@@ -104,6 +104,9 @@ void fstk_DumpCurrent(void)
 
 struct FileStackNode *fstk_GetFileStack(void)
 {
+	if (!contextStack)
+		return NULL;
+
 	struct FileStackNode *node = contextStack->fileInfo;
 
 	/* Mark node and all of its parents as referenced if not already so they don't get freed */