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/evail-fail-maxCallDepth0-maxCallDepth.err.exp

@@ -0,0 +1,6 @@
+error: stack overflow; max-call-depth exceeded
+       at /pwd/in.nix:2:22:
+            1| # Exactly two function calls deep, so should fail with maxCallDepth < 2
+            2| let f = x: x + 1; in f (f 0)
+             |                      ^
+            3|

tests/functional2/lang/function-recursion/evail-fail-maxCallDepth1-maxCallDepth.err.exp

@@ -0,0 +1,28 @@
+error:
+       … from call site
+         at /pwd/in.nix:2:22:
+            1| # Exactly two function calls deep, so should fail with maxCallDepth < 2
+            2| let f = x: x + 1; in f (f 0)
+             |                      ^
+            3|
+
+       … while calling 'f'
+         at /pwd/in.nix:2:9:
+            1| # Exactly two function calls deep, so should fail with maxCallDepth < 2
+            2| let f = x: x + 1; in f (f 0)
+             |         ^
+            3|
+
+       … while evaluating x
+         at /pwd/in.nix:2:12:
+            1| # Exactly two function calls deep, so should fail with maxCallDepth < 2
+            2| let f = x: x + 1; in f (f 0)
+             |            ^
+            3|
+
+       error: stack overflow; max-call-depth exceeded
+       at /pwd/in.nix:2:25:
+            1| # Exactly two function calls deep, so should fail with maxCallDepth < 2
+            2| let f = x: x + 1; in f (f 0)
+             |                         ^
+            3|

tests/functional2/lang/function-recursion/eval-fail-infrec.err.exp

@@ -29,7 +29,7 @@ error:
              |              ^
             2|
 
-       (19997 duplicate frames omitted)
+       (19995 duplicate frames omitted)
 
        error: stack overflow; max-call-depth exceeded
        at /pwd/in.nix:1:14:

tests/functional2/lang/function-recursion/eval-okay-maxCallDepth.out.exp

@@ -0,0 +1 @@
+2

tests/functional2/lang/function-recursion/in-maxCallDepth.nix

@@ -0,0 +1,2 @@
+# Exactly two function calls deep, so should fail with maxCallDepth < 2
+let f = x: x + 1; in f (f 0)

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"]