Suppress gcc 10 warning.

--- subversion-1.14.0/subversion/tests/svn_test.h.testwarn
+++ subversion-1.14.0/subversion/tests/svn_test.h
@@ -128,7 +128,7 @@
       return svn_error_createf(SVN_ERR_TEST_FAILED, NULL,           \
           "Strings not equal\n  Expected: '%s'\n  Found:    '%s'"   \
           "\n  at %s:%d",                                           \
-          tst_str2, tst_str1, __FILE__, __LINE__);                  \
+          tst_str2 ? tst_str2 : "(NULL)", tst_str1 ? tst_str1 : "(NULL)", __FILE__, __LINE__); \
   } while(0)
 
  /** Handy macro for testing integer equality.