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>

lib/strto.c

@@ -189,11 +189,12 @@ long trailing_strtoln(const char *str, const char *end)
 
 	if (!end)
 		end = str + strlen(str);
-	if (isdigit(end[-1])) {
-		for (p = end - 1; p > str; p--) {
-			if (!isdigit(*p))
-				return dectoul(p + 1, NULL);
-		}
+	p = end - 1;
+	if (p > str && isdigit(*p)) {
+		do {
+			if (!isdigit(p[-1]))
+				return dectoul(p, NULL);
+		} while (--p > str);
 	}
 
 	return -1;