ubsan triggered for a unit test in mduptab_add because blen was SIZE_MAX

and adding it overflowed the pointer. So add a check for that in
bstr_diff2.

bstr_diff2.c

@@ -13,6 +13,12 @@ int bstr_diff2(const char* a,const char* b,size_t blen) {
     A=a+5+uint32_read(a+1);
     a+=5;
   }
+  // If adding blen to b would overflow, reduce blen.
+  // The comparison can never get that far because the end of the
+  // address space is reserved for the kernel. Make sure to revisit this
+  // if you want to use this function in kernel space.
+  uintptr_t c = ~(uintptr_t)b;
+  if (blen > c) blen = c;
   B=b+blen;
   for (;;) {
     if (a==A) {