rvb: Refactor to address a bogus compiler warning

gcc 13 complains that we used a pointer from before a realloc call, but
it did not understand that we actually used it to calculate an offset
only.

Refactor the code to avoid this ambiguity.

Ref #7

See https://gitlab.com/uplex/varnish/libvmod-re/-/issues/7#note_2328380883

src/rvb.h

@@ -182,7 +182,7 @@ rvb_size(const struct rvb *b)
 static void
 rvb_grow(struct rvb *b, size_t l)
 {
-	const char *o;
+	ssize_t r_off, w_off;
 
 	rvb_assert(b);
 	assert(b->flags & RVB_F_MALLOC);
@@ -192,18 +192,17 @@ rvb_grow(struct rvb *b, size_t l)
 	else
 		l += rvb_size(b);
 
-	o = b->u.ro.p;
+	// prepare for relocation
+	r_off = b->u.ro.r - b->u.ro.p;
+	w_off = b->u.wr.w - b->u.wr.p;
+	assert(r_off >= 0);
+	assert(w_off >= 0);
 
 	b->u.wr.p = realloc(b->u.wr.p, l);
 	AN(b->u.ro.p);
 	b->u.ro.l = b->u.ro.p + l;
-
-	if (b->u.ro.p == o)
-		return;
-
-	// realloc has relocated
-	b->u.ro.r = b->u.ro.p + (b->u.ro.r - o);
-	b->u.wr.w = b->u.wr.p + (b->u.ro.w - o);
+	b->u.ro.r = b->u.ro.p + r_off;
+	b->u.wr.w = b->u.wr.p + w_off;
 }
 
 static void