avcodec/htmlsubtitles: Avoid locale dependant isdigit()

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b94cf549e2)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/htmlsubtitles.c

@@ -55,7 +55,7 @@ static int scanbraces(const char* in) {
     if (strncmp(in, "{\\an", 4) != 0) {
         return 0;
     }
-    if (!isdigit(in[4])) {
+    if (!av_isdigit(in[4])) {
         return 0;
     }
     if (in[5] != '}') {