fixes nim-lang#18070;fixes memory leak with nested Exceptions

compiler/ccgstmts.nim

@@ -1292,6 +1292,7 @@ proc genTryGoto(p: BProc; t: PNode; d: var TLoc) =
  # we handled the exception, remember this:
  linefmt(p, cpsStmts, "*nimErr_ = NIM_FALSE;$n", [])
  expr(p, t[i][0], d)
+ linefmt(p, cpsStmts, "#popCurrentException();$n", [])
  else:
  var orExpr = newRopeAppender()
  for j in 0..<t[i].len - 1:
@@ -1311,13 +1312,14 @@ proc genTryGoto(p: BProc; t: PNode; d: var TLoc) =
  # we handled the exception, remember this:
  linefmt(p, cpsStmts, "*nimErr_ = NIM_FALSE;$n", [])
  expr(p, t[i][^1], d)
+ linefmt(p, cpsStmts, "#popCurrentException();$n", [])
 
- linefmt(p, cpsStmts, "#popCurrentException();$n", [])
  linefmt(p, cpsStmts, "LA$1_:;$n", [nextExcept])
  endBlock(p)
 
  inc(i)
  discard pop(p.nestedTryStmts)
+
  endBlock(p)
 
  if i < t.len and t[i].kind == nkFinally:
@@ -1338,8 +1340,10 @@ proc genTryGoto(p: BProc; t: PNode; d: var TLoc) =
  # 3. finally is run for exception handling code without any 'except'
  # handler present or only handlers that did not match.
  linefmt(p, cpsStmts, "*nimErr_ = oldNimErrFin$1_;$n", [lab])
+
  endBlock(p)
  raiseExit(p)
+ linefmt(p, cpsStmts, "*nimErr_ = NIM_FALSE;$n", [])
  if hasExcept: inc p.withinTryWithExcept
 
 proc genTrySetjmp(p: BProc, t: PNode, d: var TLoc) =

tests/exception/t18070.nim

@@ -0,0 +1,18 @@
+discard """
+ matrix: "--mm:refc; --mm:orc"
+"""
+
+# bug #18070
+proc main() =
+ try:
+ try:
+ raise newException(CatchableError, "something")
+ except:
+ raise newException(CatchableError, "something else")
+ except:
+ doAssert getCurrentExceptionMsg() == "something else"
+
+ let msg = getCurrentExceptionMsg()
+ doAssert msg == "", "expected empty string but got: " & $msg
+
+main()