Fix precedence of operations in mythmpeg2.

The latest clang19 ARM compiler is pointing out that an "and"
operation has lower precedence than an "equality" operation.  Add
parentheses to ensure this expression is parsed properly.

This was introduced in 4427b93.

mythtv/libs/libmythmpeg2/header.c

@@ -625,9 +625,9 @@ static int picture_coding_ext (mpeg2dec_t * mpeg2dec)
 	    picture->nb_fields = (buffer[3] & 2) ? 3 : 2;
 	    flags |= (buffer[3] & 128) ? PIC_FLAG_TOP_FIELD_FIRST : 0;
 	} else {
-            if (buffer[3]&2 == 0)
+            if ((buffer[3]&2) == 0)
                 picture->nb_fields = 2;
-            else if (buffer[3]&128 == 0)
+            else if ((buffer[3]&128) == 0)
                 picture->nb_fields = 4;
             else
                 picture->nb_fields = 6;