avconv_opt: Consistently iterate through hwaccels array in all cases

avconv_opt.c:188:19: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]

avconv_opt.c

@@ -190,7 +190,7 @@ static int show_hwaccels(void *optctx, const char *opt, const char *arg)
     int i;
 
     printf("Supported hardware acceleration:\n");
-    for (i = 0; i < FF_ARRAY_ELEMS(hwaccels) - 1; i++) {
+    for (i = 0; hwaccels[i].name; i++) {
         printf("%s\n", hwaccels[i].name);
     }
     printf("\n");