[perl #121484] /m causing false negative

My recent commit d0d4464 in re_intuit_start() reduced the
scope of a 'skip if multiline' check, so that certain optimisations
weren't being unnecessarily skipped. Unfortunately it didn't reduce the
scope enough, so a vital slen-- was being skipped in the
SvTAIL-but-don't-fail case.

This commit just moves the !multiline test further down, and updates the
commentary and condition formatting a bit.

regexec.c

@@ -804,12 +804,15 @@ Perl_re_intuit_start(pTHX_
                     "  Looking for check substr at fixed offset %"IVdf"...\n",
                     (IV)prog->check_offset_min));
 
-	        if (SvTAIL(check) && !multiline) {
-                    /* In this case, the regex is anchored at the end too,
-                     * so the lengths must match exactly, give or take a \n.
-		     * NB: slen >= 1 since the last char of check is \n */
-		    if (   strend - s > slen || strend - s < slen - 1
-		       || (strend - s == slen && strend[-1] != '\n'))
+	        if (SvTAIL(check)) {
+                    /* In this case, the regex is anchored at the end too.
+                     * Unless it's a multiline match, the lengths must match
+                     * exactly, give or take a \n.  NB: slen >= 1 since
+                     * the last char of check is \n */
+		    if (!multiline
+                        && (   strend - s > slen
+                            || strend - s < slen - 1
+                            || (strend - s == slen && strend[-1] != '\n')))
                     {
                         DEBUG_EXECUTE_r(PerlIO_printf(Perl_debug_log,
                                             "  String too long...\n"));

t/re/re_tests

@@ -1883,6 +1883,7 @@ A+(*PRUNE)BC(?{})	AAABC	y	$&	AAABC
 \d<(.*?)>	a<>	n	-	-
 [bcd].{2,3}aaaa	XbXaaaaa	y	-	-
 [bcd].{2,3}aaaa	Xb\x{100}aaaaa	y	-	-
+'\Awibble\z'm	wibble	y	-	-
 
 # Keep these lines at the end of the file
 # vim: softtabstop=0 noexpandtab