Avoid undefined behaviour in DOH Replace() function

If the source and replacement strings were the same length, the code
was performing undefined pointer arithmetic involving a NULL pointer.
I'm not aware of any observable effects of this in practice, but it's
potentially problematic.  It's detected by ubsan, for example when
running `make check-python-test-suite`:

DOH/string.c:839:4: runtime error: applying non-zero offset to non-null pointer 0x602000001558 produced null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior DOH/string.c:839:4 in

1 files changed, 3 insertions, 1 deletions

diff --git a/Source/DOH/string.c b/Source/DOH/string.c
index 3689f4ffe..093330b89 100644
--- a/Source/DOH/string.c
+++ b/Source/DOH/string.c
@@ -836,7 +836,9 @@ static int replace_simple(String *str, char *token, char *rep, int flags, int co
 	  memmove(t, s, (str->str + str->len) - s + 1);
 	}
       } else {
-	t += (c - s);
+	if (c) {
+	  t += (c - s);
+	}
       }
       s = c;
       ic--;