Allow -2147483648 (LONG_MIN) as valid input to strtol(), not setting errno.

This is what other implementations do and seems to be the intent of the standards, although the wording isn't entirely clear.

stdlib.asm

@@ -843,10 +843,16 @@ cn3      ph4   str                      save the starting string
          sta   val
          txy                            see if we have an overflow
          bpl   rt1
+         ldy   negative                   allow -2147483648 as legal value
+         beq   ov0
+         cpx   #$8000
+         bne   ov0
+         tay
+         beq   rt1
 ;
 ;  Overflow - flag the error
 ;
-         lda   #ERANGE                  errno = ERANGE
+ov0      lda   #ERANGE                  errno = ERANGE
          sta   >errno
          ldx   #$7FFF                   return value = LONG_MAX
          ldy   #$FFFF