strtoul: return 0 when given an empty string, as required by the C standards.

The issue was introduced in commit 6e9790667.

errno is now set to EINVAL in this case. (This is not required by the C standards, but is consistent with various other implementations.)

This fixes the cases in #23 related to strtol() and strtoul().

stdlib.asm

@@ -926,6 +926,9 @@ cn4      inc4  str                      next char
 
 cn5      lda   foundOne                 if no digits were found, flag the error
          bne   rt1
+         lda   #EINVAL
+         sta   >errno
+         bra   rt2
 ;
 ;  flag an error
 ;