ffprobe/flat: add escape for '`' and '$'.

ffprobe.c

@@ -806,6 +806,8 @@ static const char *flat_escape_value_str(AVBPrint *dst, const char *src)
         case '\r': av_bprintf(dst, "%s", "\\r");  break;
         case '\\': av_bprintf(dst, "%s", "\\\\"); break;
         case '"':  av_bprintf(dst, "%s", "\\\""); break;
+        case '`':  av_bprintf(dst, "%s", "\\`");  break;
+        case '$':  av_bprintf(dst, "%s", "\\$");  break;
         default:   av_bprint_chars(dst, *p, 1);   break;
         }
     }