Make invalid UTF-8 characters in strings non-fatal

STRLEN and STRSUB report the erroneous bytes

Fixes #848

test/asm/invalid-utf-8-strings.asm

@@ -0,0 +1,23 @@
+; characters:
+; 1: U+0061 a
+; 2: U+00E4 a with diaresis (0xC3 0xA4)
+; 3: U+0062 b
+; 4: U+6F22 kanji (0xE6 0xBC 0xA2)
+; 5: U+002C ,
+; 6: U+0061 a
+; 7: invalid byte 0xA3
+; 8: invalid byte 0xA4
+; 9: U+0062 b
+; 10: invalid bytes 0xE6 0xF0
+; 11: invalid byte 0xA2
+; 12: U+0021 !
+invalid EQUS "aäb漢,a<><61>b<EFBFBD><62><EFBFBD>!"
+
+n = STRLEN("{invalid}")
+copy EQUS STRSUB("{invalid}", 1)
+
+println "\"{invalid}\" == \"{copy}\" ({d:n})"
+
+mid1 EQUS STRSUB("{invalid}", 5, 2)
+mid2 EQUS STRSUB("{invalid}", 9, 1)
+println "\"{mid2}{mid1}\""