[PATCH 2/3] wildmatch: avoid undefined behavior

[Phillip Wood <phillip.wood123@gmail.com>]

From: Phillip Wood <phillip.wood@dunelm.org.uk>

The code changed in this commit is designed to check if the pattern
starts with "**/" or contains "/**/" (see 3a078dec33 (wildmatch: fix
"**" special case, 2013-01-01)). Unfortunately when the pattern begins
with "**/" `prev_p = p - 2` is evaluated when `p` points to the second
"*" and so the subtraction is undefined according to section 6.5.6 of
the C standard because the result does not point within the same object
as `p`. Fix this by avoiding the subtraction unless it is well defined.

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
---
 wildmatch.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/wildmatch.c b/wildmatch.c
index 06861bd8bc..694d2f8e40 100644
--- a/wildmatch.c
+++ b/wildmatch.c
@@ -83,12 +83,12 @@ static int dowild(const uchar *p, const uchar *text, unsigned int flags)
 			continue;
 		case '*':
 			if (*++p == '*') {
-				const uchar *prev_p = p - 2;
+				const uchar *prev_p = p;
 				while (*++p == '*') {}
 				if (!(flags & WM_PATHNAME))
 					/* without WM_PATHNAME, '*' == '**' */
 					match_slash = 1;
-				else if ((prev_p < pattern || *prev_p == '/') &&
+				else if ((prev_p - pattern < 2 || *(prev_p - 2) == '/') &&
 				    (*p == '\0' || *p == '/' ||
 				     (p[0] == '\\' && p[1] == '/'))) {
 					/*
-- 
2.39.2