Don't walk off front of string when stripping newline

If the string was zero-length to begin with, or consists of only newline
characters, nothing stopped us from incrementing right off the front of
the string. Ensure len stays above zero the whole time.

Signed-off-by: Dan McGee <dan@archlinux.org>

lib/libalpm/util.c

@@ -219,7 +219,7 @@ size_t _alpm_strip_newline(char *str)
 		return 0;
 	}
 	len = strlen(str);
-	while(str[len - 1] == '\n') {
+	while(len > 0 && str[len - 1] == '\n') {
 		len--;
 	}
 	str[len] = '\0';