Avoid bogus -Wmaybe-uninitialized from gcc 11

It doesn't really matter that number_buf is not initialized because its
first characters are not going to be used anyhow (png_format_number()
returns a pointer to the actually used part of the string at the end),
but gcc-11 warns about it, so do initialize it just to make it happy.

png.c

@@ -752,7 +752,7 @@ png_convert_to_rfc1123_buffer(char out[29], png_const_timep ptime)
 
    {
       size_t pos = 0;
-      char number_buf[5]; /* enough for a four-digit year */
+      char number_buf[5] = {0}; /* enough for a four-digit year */
 
 #     define APPEND_STRING(string) pos = png_safecat(out, 29, pos, (string))
 #     define APPEND_NUMBER(format, value)\