Make url_read_complete retry on EAGAIN and return how much data it read

if it reached EOF, making it useful in more cases.

Originally committed as revision 21393 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avio.c

@@ -156,8 +156,10 @@ int url_read_complete(URLContext *h, unsigned char *buf, int size)
     len = 0;
     while (len < size) {
         ret = url_read(h, buf+len, size-len);
-        if (ret < 1)
-            return ret;
+        if (ret == AVERROR(EAGAIN)) {
+            ret = 0;
+        } else if (ret < 1)
+            return ret < 0 ? ret : len;
         len += ret;
     }
     return len;