fix(sem): missed discard in try/finally errors (nim-works#995)

## Summary

A missing `discard` in a `try/finally` no longer crashes the compiler
and emits an error as it should.

## Details

The `discardCheck` used to ensure expressions are used or `void` did not
traverse `try` appropriately. Instead of looking at the last expression
in the last `except` or `try` branch, it would look at the last branch,
even if that was a `finally`. This led to incomplete data being sent for
error reporting, which ultimately led to an NPE.

Along with the fix a regression test has been added.

compiler/sem/semstmts.nim

@@ -135,7 +135,18 @@ proc discardCheck(c: PContext, n: PNode, flags: TExprFlags): PNode =
     elif n.typ.kind != tyError and c.config.cmd != cmdInteractive:
       var m = n
       while m.kind in skipForDiscardable:
-        m = m.lastSon
+        case m.kind
+        of nkTryStmt:
+          m =
+            case m[^1].kind
+            of nkFinally:
+              m[^2]
+            of nkExceptBranch:
+              m[^1]
+            of nkAllNodeKinds - {nkFinally, nkExceptBranch}:
+              unreachable()
+        else:
+          m = m.lastSon
 
       result = newError(c.config, n,
         PAstDiag(kind: adSemUseOrDiscardExpr, undiscarded: m))
@@ -299,7 +310,6 @@ proc semTry(c: PContext, n: PNode; flags: TExprFlags): PNode =
       n[i][^1] = discardCheck(c, n[i][^1], flags)
       if n[i][^1].isError:
         return wrapError(c.config, n)
-
     if typ == c.enforceVoidContext:
       result.typ = c.enforceVoidContext
   else:

tests/lang_exprs/tdiscardcheck_try_except.nim

@@ -0,0 +1,14 @@
+discard """
+description: "Ensure discard checks are done for try expressions"
+errormsg: "expression '1' is of type 'int literal(1)' and has to be used (or discarded)"
+line: 14
+"""
+
+## This was originally introduced as a regression test, where a fix to
+## `semstmts.discardCheck` erroneously changed traversals in a manner where
+## the test suite passed but this was not caught, except in code review.
+
+try:
+  discard
+except:
+  1

tests/lang_exprs/tdiscardcheck_try_finally.nim

@@ -0,0 +1,16 @@
+discard """
+description: "Ensure discard checks are done for try expressions"
+errormsg: "expression '1' is of type 'int literal(1)' and has to be used (or discarded)"
+line: 14
+"""
+
+## This was originally introduced as a regression test, where
+## `semstmts.discardCheck` erroneously checked the `finally` branch, instead
+## of the last non-`finally` branch in the `try` expression. This generated an
+## incomplete report, and resulted in an NPE within `cli_reporter` when
+## attempting to render it.
+
+try:
+  1
+finally:
+  discard 1