[Msan] Fix strlen() and strnlen() interceptors to work on FreeBSD

[eugenis]

LGTM

lib/msan/msan_interceptors.cc

INTERCEPTOR(char *, stpcpy, char *dest, const char *src) {  // NOLINT
   char *res = REAL(stpcpy)(dest, src);  // NOLINT
   CopyPoison(dest, src, n + 1, &stack);
   return res;
 }
 
 INTERCEPTOR(char *, strdup, char *src) {
   ENSURE_MSAN_INITED();
   GET_STORE_STACK_TRACE;
+  // On FreeBSD strdup() leverages strlen().
+  InterceptorScope interceptor_scope;
   SIZE_T n = REAL(strlen)(src);
   char *res = REAL(strdup)(src);
   CopyPoison(res, src, n + 1, &stack);
   return res;
 }
 
 #if !SANITIZER_FREEBSD
 INTERCEPTOR(char *, __strdup, char *src) {
   ENSURE_MSAN_INITED();
   GET_STORE_STACK_TRACE;
   SIZE_T n = REAL(strlen)(src);
   char *res = REAL(__strdup)(src);
   CopyPoison(res, src, n + 1, &stack);
   return res;
 }
 #define MSAN_MAYBE_INTERCEPT___STRDUP INTERCEPT_FUNCTION(__strdup)
 #else
 #define MSAN_MAYBE_INTERCEPT___STRDUP
 #endif
 
 INTERCEPTOR(char *, strndup, char *src, SIZE_T n) {
   ENSURE_MSAN_INITED();
   GET_STORE_STACK_TRACE;
+  // On FreeBSD strndup() leverages strnlen().
+  InterceptorScope interceptor_scope;
   SIZE_T copy_size = REAL(strnlen)(src, n);
   char *res = REAL(strndup)(src, n);
   CopyPoison(res, src, copy_size, &stack);
   __msan_unpoison(res + copy_size, 1); // \0
   return res;
 }
 
 #if !SANITIZER_FREEBSD