STRSUB and CHARSUB allow zero or negative positions

These are offsets from the end of the string, as if the
STRLEN or CHARLEN respectively were added to the position.

Fixes #812

test/asm/strsub.out

@@ -1,6 +1,9 @@
 A
 B
 C
+A
+B
+C
 AB
 BC
 BC