mach: strerror must not return NULL (bug 30555)

This follows 1d44530a5b ("string: strerror must not return NULL (bug 30555)"):

«
    For strerror, this fixes commit 28aff04781 ("string:
    Implement strerror in terms of strerror_l").  This commit avoids
    returning NULL for strerror_l as well, although POSIX allows this
    behavior for strerror_l.
»

sysdeps/mach/strerror_l.c

@@ -61,11 +61,18 @@ __strerror_l (int errnum, locale_t loc)
       free (tls_internal->strerror_l_buf);
       if (__asprintf (&tls_internal->strerror_l_buf, "%s%X",
 		      translate ("Error in unknown error system: ", loc),
-		      errnum) == -1)
-	tls_internal->strerror_l_buf = NULL;
+		      errnum) > 0)
+	err = tls_internal->strerror_l_buf;
+      else
+	{
+	  /* The memory was freed above.  */
+	  tls_internal->strerror_l_buf = NULL;
+	  /* Provide a fallback translation.  */
+	  err = (char *) translate ("Unknown error", loc);
+	}
 
       __set_errno (saved_errno);
-      return tls_internal->strerror_l_buf;
+      return err;
     }
 
   es = &__mach_error_systems[system];
@@ -79,10 +86,15 @@ __strerror_l (int errnum, locale_t loc)
       if (__asprintf (&tls_internal->strerror_l_buf, "%s%s %d",
 		      translate ("Unknown error ", loc),
 		      translate (es->subsystem[sub].subsys_name, loc),
-		      errnum) == -1)
-	tls_internal->strerror_l_buf = NULL;
-
-      err = tls_internal->strerror_l_buf;
+		      errnum) > 0)
+	err = tls_internal->strerror_l_buf;
+      else
+	{
+	  /* The memory was freed above.  */
+	  tls_internal->strerror_l_buf = NULL;
+	  /* Provide a fallback translation.  */
+	  err = (char *) translate ("Unknown error", loc);
+	}
     }
   else
     err = (char *) translate (es->subsystem[sub].codes[code], loc);