lib: Fix a few bugs in trailing_strtoln()

At present this has a minor bug in that it reads the byte before the
start of the string, if it is empty. Also it doesn't handle a
non-numeric prefix which is only one character long.

Fix these bugs with a reworked implementation. Add a test for the second
case. The first one is hard to test.

Signed-off-by: Simon Glass <sjg@chromium.org>

test/str_ut.c

@@ -257,6 +257,8 @@ static int str_trailing(struct unit_test_state *uts)
 	ut_asserteq(123, trailing_strtoln(str1, str1 + 6));
 	ut_asserteq(-1, trailing_strtoln(str1, str1 + 9));
 
+	ut_asserteq(3, trailing_strtol("a3"));
+
 	return 0;
 }
 STR_TEST(str_trailing, 0);