avformat/flvenc: Avoid avio_write(pb, "", 0)

When the compiler chooses to inline put_amf_string(pb, ""),
the avio_write(pb, "", 0) can be avoided. Happens with
Clang-17 with -O1 and higher and GCC 13 with -O2 and higher
here.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/flvenc.c

@@ -231,6 +231,9 @@ static void put_amf_string(AVIOContext *pb, const char *str)
 {
     size_t len = strlen(str);
     avio_wb16(pb, len);
+    // Avoid avio_write() if put_amf_string(pb, "") is inlined.
+    if (av_builtin_constant_p(len == 0) && len == 0)
+        return;
     avio_write(pb, str, len);
 }