BUG/MINOR: map: fix map_regm with backref

Due to a cascade of get_trash_chunk calls the sample is
corrupted when we want to read it.

The fix consist to use a temporary chunk to copy the sample
value and use it.

[wt: for 1.8 and older, a backport was successfully tested here :
 https://www.mail-archive.com/haproxy@formilux.org/msg30694.html]

src/map.c

@@ -185,11 +185,26 @@ static int sample_conv_map(const struct arg *arg_p, struct sample *smp, void *pr
 		if (pat->data) {
 			/* In the regm case, merge the sample with the input. */
 			if ((long)private == PAT_MATCH_REGM) {
+				struct buffer *tmptrash;
+
+				/* Copy the content of the sample because it could
+				   be scratched by incoming get_trash_chunk */
+				tmptrash = alloc_trash_chunk();
+				if (!tmptrash)
+					return 0;
+
+				tmptrash->data = smp->data.u.str.data;
+				if (tmptrash->data > (tmptrash->size-1))
+					tmptrash->data = tmptrash->size-1;
+
+				memcpy(tmptrash->area, smp->data.u.str.area, tmptrash->data);
+				tmptrash->area[tmptrash->data] = 0;
+
 				str = get_trash_chunk();
 				str->data = exp_replace(str->area, str->size,
-				                       smp->data.u.str.area,
-				                       pat->data->u.str.area,
-				                       (regmatch_t *)smp->ctx.a[0]);
+				                        tmptrash->area,
+				                        pat->data->u.str.area,
+				                        (regmatch_t *)smp->ctx.a[0]);
 				if (str->data == -1)
 					return 0;
 				smp->data.u.str = *str;