grep.c: do "if (bool && memchr())" not "if (memchr() && bool)"

Change code in compile_regexp() to check the cheaper boolean
"!opt->pcre2" condition before the "memchr()" search.

This doesn't noticeably optimize anything, but makes the code more
obvious and conventional. The line wrapping being added here also
makes a subsequent commit smaller.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

grep.c

@@ -492,7 +492,8 @@ static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 	p->ignore_case = opt->ignore_case;
 	p->fixed = opt->fixed;
 
-	if (memchr(p->pattern, 0, p->patternlen) && !opt->pcre2)
+	if (!opt->pcre2 &&
+	    memchr(p->pattern, 0, p->patternlen))
 		die(_("given pattern contains NULL byte (via -f <file>). This is only supported with -P under PCRE v2"));
 
 	p->is_fixed = is_fixed(p->pattern, p->patternlen);