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-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|