Fix unlikely pointer overflow in abspath

* src/function.c (abspath): len is now ptrdiff_t,
to avoid GCC warning about comparing signed to unsigned.
It really is a pointer difference, after all.
Rejigger comparision to avoid undefined behavior
if dest + len is an invalid pointer.

src/function.c

diff --git a/src/function.c b/src/function.c
index 133e33a..b4c3805 100644
--- a/src/function.c
+++ b/src/function.c
@@ -2119,7 +2119,7 @@
 
   for (start = end = name; *start != '\0'; start = end)
     {
-      size_t len;
+      ptrdiff_t len;
 
       /* Skip sequence of multiple path-separators.  */
       while (ISDIRSEP (*start))
@@ -2147,7 +2147,7 @@
           if (! ISDIRSEP (dest[-1]))
             *dest++ = '/';
 
-          if (dest + len >= apath_limit)
+          if (apath_limit - dest <= len)
             return NULL;
 
           dest = mempcpy (dest, start, len);