BUG/MEDIUM: resolvers: fix truncated TLD consecutive to the API fix

A bug was introduced by commit previous bf9498a31 ("MINOR: resolvers:
fix the resolv_str_to_dn_label() API about trailing zero") as the code
is particularly contrived and hard to test. The output writes the last
char at [i+1] so the trailing zero and return value must be at i+1.

This will have to be backported where the patch above is backported
since it was needed for a fix.

(cherry picked from commit 7b232f132dc9a0a6ee6fa3fabfa6e50196af9240)
Signed-off-by: Willy Tarreau <w@1wt.eu>
diff --git a/src/resolvers.c b/src/resolvers.c
index e7a2516..b51aaa2 100644
--- a/src/resolvers.c
+++ b/src/resolvers.c
@@ -1690,8 +1690,8 @@
 		dn[i+1] = str[i];
 	}
 	dn[offset] = i - offset;
-	dn[i] = '\0';
-	return i;
+	dn[i+1] = '\0';
+	return i+1;
 }
 
 /* Validates host name: