die_with_error: Save errno sooner

We need to save errno immediately, otherwise it could be overwritten
by a failing library call somewhere in the implementation of fprintf.

Signed-off-by: Simon McVittie <smcv@collabora.com>

utils.c

@@ -56,10 +56,10 @@ die_with_error (const char *format, ...)
   va_list args;
   int errsv;
 
-  fprintf (stderr, "bwrap: ");
-
   errsv = errno;
 
+  fprintf (stderr, "bwrap: ");
+
   va_start (args, format);
   vfprintf (stderr, format, args);
   va_end (args);