Revert some uses of xsprintf

This reverts some changes introduced in d054f0a4d4.
xsprintf should be used in cases where we calculated the right buffer
size by hand (using DECIMAL_STRING_MAX and such), and never in cases where
we are printing externally specified strings of arbitrary length.

Fixes #4534.

src/basic/log.c

@@ -782,7 +782,7 @@ static void log_assert(
                 return;
 
         DISABLE_WARNING_FORMAT_NONLITERAL;
-        xsprintf(buffer, format, text, file, line, func);
+        snprintf(buffer, sizeof buffer, format, text, file, line, func);
         REENABLE_WARNING;
 
         log_abort_msg = buffer;