helpers.c: Fix undefined behavior in strndup()

Using strlen() or strdup() inside strndup() is problematic: if the
input string is not null-terminated, these functions can read past the
end of the buffer, which triggers undefined behavior. Rewrite the
function to never read past the provided `maxlen` bound.

Change-Id: Id34127024085879228626fbad59af03268ec5255
Signed-off-by: Xiang Wang <merle@hardenedliux.org>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/49741
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/helpers.c b/helpers.c
index c83cd2c..289848d 100644
--- a/helpers.c
+++ b/helpers.c
@@ -106,15 +106,16 @@
 /* strndup is a POSIX function not present in MinGW */
 char *strndup(const char *src, size_t maxlen)
 {
-	if (strlen(src) > maxlen) {
-		char *retbuf;
-		if ((retbuf = malloc(1 + maxlen)) != NULL) {
-			memcpy(retbuf, src, maxlen);
-			retbuf[maxlen] = '\0';
-		}
-		return retbuf;
+	char *retbuf;
+	size_t len;
+	for (len = 0; len < maxlen; len++)
+		if (src[len] == '\0')
+			break;
+	if ((retbuf = malloc(1 + len)) != NULL) {
+		memcpy(retbuf, src, len);
+		retbuf[len] = '\0';
 	}
-	return strdup(src);
+	return retbuf;
 }
 #endif