ffprobe: make upcase_string() ignore non-ASCII characters

This is required as some section names may contain non-ASCII characters
(e.g. '_').

ffprobe.c

@@ -417,7 +417,11 @@ static inline char *upcase_string(char *dst, size_t dst_size, const char *src)
 {
     int i;
     for (i = 0; src[i] && i < dst_size-1; i++)
+        if (src[i] >= 'a' && src[i] <= 'z') {
             dst[i] = src[i]-32;
+        } else {
+            dst[i] = src[i];
+        }
     dst[i] = 0;
     return dst;
 }