avcodec/videotoolboxenc: fix undefined behavior with rc_max_rate=0

On macOS, a zero rc_max_rate cause an error from
VTSessionSetProperty(kVTCompressionPropertyKey_DataRateLimits).

on iOS (depending on device/version), a zero rc_max_rate cause invalid
arguments from the vtenc_output_callback after few frames and then a crash
within the VideoToolbox library.
Signed-off-by: Aman Gupta <aman@tmm1.net>

libavcodec/videotoolboxenc.c

@@ -1017,7 +1017,7 @@ static int vtenc_create_encoder(AVCodecContext   *avctx,
         return AVERROR_EXTERNAL;
     }
 
-    if (vtctx->codec_id == AV_CODEC_ID_H264) {
+    if (vtctx->codec_id == AV_CODEC_ID_H264 && max_rate > 0) {
         // kVTCompressionPropertyKey_DataRateLimits is not available for HEVC
         bytes_per_second_value = max_rate >> 3;
         bytes_per_second = CFNumberCreate(kCFAllocatorDefault,
@@ -1058,7 +1058,10 @@ static int vtenc_create_encoder(AVCodecContext   *avctx,
             av_log(avctx, AV_LOG_ERROR, "Error setting max bitrate property: %d\n", status);
             return AVERROR_EXTERNAL;
         }
+    }
 
+    if (vtctx->codec_id == AV_CODEC_ID_H264) {
+        // kVTCompressionPropertyKey_ProfileLevel is not available for HEVC
         if (profile_level) {
             status = VTSessionSetProperty(vtctx->session,
                                         kVTCompressionPropertyKey_ProfileLevel,