lavu/opt: fix av_opt_get_key_value() API.

Do not skip the end delimiter.
Reserve positive return values.
This is an API break, but the function was introduced less than
two weeks ago.

cmdutils.c

@@ -582,6 +582,8 @@ static int init_report(const char *env)
                        av_err2str(ret));
             break;
         }
+        if (*env)
+            env++;
         count++;
         if (!strcmp(key, "file")) {
             filename_template = val;