compat/regex: explicitly mark intentional use of the comma operator

The comma operator is a somewhat obscure C feature that is often used by
mistake and can even cause unintentional code flow. That is why the
`-Wcomma` option of clang was introduced: To identify unintentional uses
of the comma operator.

In the `compat/regex/` code, the comma operator is used twice, once to
avoid surrounding two conditional statements with curly brackets, the
other one to increment two counters simultaneously in a `do ... while`
condition.

The first one is replaced with a proper conditional block, surrounded by
curly brackets.

The second one would be harder to replace because the loop contains two
`continue`s. Therefore, the second one is marked as intentional by
casting the value-to-discard to `void`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

compat/regex/regex_internal.c

@@ -1232,7 +1232,10 @@ re_node_set_merge (re_node_set *dest, const re_node_set *src)
        is = src->nelem - 1, id = dest->nelem - 1; is >= 0 && id >= 0; )
     {
       if (dest->elems[id] == src->elems[is])
-	is--, id--;
+	{
+	  is--;
+	  id--;
+	}
       else if (dest->elems[id] < src->elems[is])
 	dest->elems[--sbase] = src->elems[is--];
       else /* if (dest->elems[id] > src->elems[is]) */

compat/regex/regexec.c

@@ -2210,7 +2210,7 @@ sift_states_bkref (const re_match_context_t *mctx, re_sift_context_t *sctx,
 	  /* mctx->bkref_ents may have changed, reload the pointer.  */
 	  entry = mctx->bkref_ents + enabled_idx;
 	}
-      while (enabled_idx++, entry++->more);
+      while ((void)enabled_idx++, entry++->more);
     }
   err = REG_NOERROR;
  free_return: