Add len check for newIfStmt to avoid segfault (#11032)

(cherry picked from commit a68c5662f2)

lib/core/macros.nim

@@ -976,6 +976,8 @@ proc newIfStmt*(branches: varargs[tuple[cond, body: NimNode]]):
   ##    )
   ##
   result = newNimNode(nnkIfStmt)
+  if len(branches) < 1:
+    error("If statement must have at least one branch")
   for i in branches:
     result.add(newTree(nnkElifBranch, i.cond, i.body))