src/libexpr/search-path.cc: avoid out-of-bounds read on string_view

Without the change build with `-D_GLIBCXX_ASSERTIONS` exposes testsuite
assertion:

    $ gdb src/libexpr/tests/libnixexpr-tests
    Reading symbols from src/libexpr/tests/libnixexpr-tests...
    (gdb) break __glibcxx_assert_fail
    (gdb) run
    (gdb) bt
    in std::__glibcxx_assert_fail(char const*, int, char const*, char const*)@plt () from /mnt/archive/big/git/nix/src/libexpr/libnixexpr.so
    in std::basic_string_view<char, std::char_traits<char> >::operator[] (this=0x7fffffff56c0, __pos=4)
        at /nix/store/r74fw2j8rx5idb0w8s1s6ynwwgs0qmh9-gcc-14.0.0/include/c++/14.0.0/string_view:258
    in nix::SearchPath::Prefix::suffixIfPotentialMatch (this=0x7fffffff5780, path=...) at src/libexpr/search-path.cc:15
    in nix::SearchPathElem_suffixIfPotentialMatch_partialPrefix_Test::TestBody (this=0x555555a17540) at src/libexpr/tests/search-path.cc:62

As string sizes are usigned types `(a - b) > 0` effectively means
`a != b`. While the intention should be `a > b`.

The change fixes test suite pass.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libexpr/search-path.cc b/src/libexpr/search-path.cc
index 36bb4c3a5..180d5f8b1 100644
--- a/src/libexpr/search-path.cc
+++ b/src/libexpr/search-path.cc
@@ -10,7 +10,7 @@ std::optional<std::string_view> SearchPath::Prefix::suffixIfPotentialMatch(
 
     /* Non-empty prefix and suffix must be separated by a /, or the
        prefix is not a valid path prefix. */
-    bool needSeparator = n > 0 && (path.size() - n) > 0;
+    bool needSeparator = n > 0 && n < path.size();
 
     if (needSeparator && path[n] != '/') {
         return std::nullopt;