libexpr/eval: Fix off by one in maxCallDepth calculation

This isn't really relevant for a setting of 10k, but now that we want to
set it to 0 for Flakes it makes a difference as to whether or not one
top-level function call is allowed or not. (It shouldn't be)

Change-Id: I6bbf99826af0289c232b9d45d172a378b81abe4b

tests/functional2/lang/function-recursion/test.toml

@@ -0,0 +1,21 @@
+[[test]]
+runner = "eval-fail"
+in = ["in-infrec.nix", "in-mutrec.nix"]
+matrix = true
+
+[[test]]
+runner = "eval-okay"
+in = "in-maxCallDepth.nix"
+flags = ["--option", "max-call-depth", "2"]
+
+[[test]]
+name = "evail-fail-maxCallDepth1"
+runner = "eval-fail"
+in = "in-maxCallDepth.nix"
+flags = ["--option", "max-call-depth", "1"]
+
+[[test]]
+name = "evail-fail-maxCallDepth0"
+runner = "eval-fail"
+in = "in-maxCallDepth.nix"
+flags = ["--option", "max-call-depth", "0"]