Fix error handling at openssl_strerror_r

When bufsize == 0, openssl_strerror_r should return 0 (if _GNU_SOURCE is defined),
to be consistent with non-_GNU_SOURCE variants, which exhibit the same behavior.
Fix a few cases, where the return value of openssl_strerror_r was ignored.
Reviewed-by: NPaul Dale <paul.dale@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9163)

(cherry picked from commit e7a4682d0b347f0dfba629f4601a28801e54ad67)

crypto/o_str.c

@@ -231,7 +231,7 @@ int openssl_strerror_r(int errnum, char *buf, size_t buflen)
      * buf is left unused.
      */
     err = strerror_r(errnum, buf, buflen);
-    if (err == NULL)
+    if (err == NULL || buflen == 0)
         return 0;
     /*
      * If err is statically allocated, err != buf and we need to copy the data.

crypto/store/loader_file.c

@@ -860,10 +860,10 @@ static OSSL_STORE_LOADER_CTX *file_open(const OSSL_STORE_LOADER *loader,
         if (ctx->_.dir.last_entry == NULL) {
             if (ctx->_.dir.last_errno != 0) {
                 char errbuf[256];
-                errno = ctx->_.dir.last_errno;
-                openssl_strerror_r(errno, errbuf, sizeof(errbuf));
                 OSSL_STOREerr(OSSL_STORE_F_FILE_OPEN, ERR_R_SYS_LIB);
-                ERR_add_error_data(1, errbuf);
+                errno = ctx->_.dir.last_errno;
+                if (openssl_strerror_r(errno, errbuf, sizeof(errbuf)))
+                    ERR_add_error_data(1, errbuf);
                 goto err;
             }
             ctx->_.dir.end_reached = 1;
@@ -1260,11 +1260,11 @@ static OSSL_STORE_INFO *file_load(OSSL_STORE_LOADER_CTX *ctx,
                 if (!ctx->_.dir.end_reached) {
                     char errbuf[256];
                     assert(ctx->_.dir.last_errno != 0);
+                    OSSL_STOREerr(OSSL_STORE_F_FILE_LOAD, ERR_R_SYS_LIB);
                     errno = ctx->_.dir.last_errno;
                     ctx->errcnt++;
-                    openssl_strerror_r(errno, errbuf, sizeof(errbuf));
-                    OSSL_STOREerr(OSSL_STORE_F_FILE_LOAD, ERR_R_SYS_LIB);
-                    ERR_add_error_data(1, errbuf);
+                    if (openssl_strerror_r(errno, errbuf, sizeof(errbuf)))
+                        ERR_add_error_data(1, errbuf);
                 }
                 return NULL;
             }