Fix ast_str_truncate signedness warning and documentation.

Review: https://reviewboard.asterisk.org/r/1594
........

Merged revisions 346144 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@346145 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/strings.h

@@ -498,14 +498,20 @@ char * attribute_pure ast_str_buffer(const struct ast_str *buf),
 
 /*!\brief Truncates the enclosed string to the given length.
  * \param buf A pointer to the ast_str structure.
- * \param len Maximum length of the string.
+ * \param len Maximum length of the string. If len is larger than the
+ *        current maximum length, things will explode. If it is negative
+ *        at most -len characters will be trimmed off the end.
  * \retval A pointer to the resulting string.
  */
 AST_INLINE_API(
 char *ast_str_truncate(struct ast_str *buf, ssize_t len),
 {
 	if (len < 0) {
-		buf->__AST_STR_USED += ((ssize_t) abs(len)) > (ssize_t) buf->__AST_STR_USED ? -buf->__AST_STR_USED : len;
+		if ((typeof(buf->__AST_STR_USED)) -len >= buf->__AST_STR_USED) {
+			buf->__AST_STR_USED = 0;
+		} else {
+			buf->__AST_STR_USED += len;
+		}
 	} else {
 		buf->__AST_STR_USED = len;
 	}