Fix string function behavior with NUL characters (#1746)

test/asm/null-char-functions.asm

@@ -0,0 +1,33 @@
+def s equs "hello\0world"
+
+println #s
+
+MACRO assert_equal
+	assert !strcmp(\1, \2)
+ENDM
+
+assert strlen(#s) == 11
+assert strfind(#s, "o\0w") == 4
+assert strfind(#s, "orld") == 7
+assert strrfind(#s, "o\0w") == 4
+assert strrfind(#s, "o") == 7
+
+assert_equal strcat(#s, "\0lol"), "hello\0world\0lol"
+assert_equal #s ++ "\0lol", "hello\0world\0lol"
+assert_equal strupr(#s), "HELLO\0WORLD"
+assert_equal strlwr("HELLO\0WORLD"), #s
+assert_equal strslice(#s, 4, 7), "o\0w"
+assert_equal strslice(#s, 6), "world"
+assert_equal strrpl(#s, "o", "XX"), "hellXX\0wXXrld"
+assert_equal strrpl(#s, "\0", "0"), "hello0world"
+assert_equal strfmt("%s", #s), #s
+assert_equal strchar(#s, 5), "\0"
+assert_equal strchar(#s, -1), "d"
+
+assert strin(#s, "o\0w") == 5
+assert strin(#s, "orld") == 8
+assert strrin(#s, "o\0w") == 5
+assert strrin(#s, "o") == 8
+
+assert_equal strsub(#s, 5, 3), "o\0w"
+assert_equal strsub(#s, 7), "world"