Properly handle partial reads from fgets() when handling AGIs.

When fgets() failed with EAGAIN, we were continually decrementing the available
space left in our buffer, resulting in botched command handling.

(closes issue #16032)
Reported by: notahat
Patches:
      agi_buffer_patch2.diff uploaded by fnordian (license 110)


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.2@302548 65c4cc65-6c06-0410-ace0-fbb531ad65f3

res/res_agi.c

@@ -2920,7 +2920,7 @@ static enum agi_result run_agi(struct ast_channel *chan, char *request, AGI *agi
 			retry = AGI_NANDFS_RETRY;
 			buf[0] = '\0';
 
-			while (buflen < (len - 1)) {
+			while (len > 1) {
 				res = fgets(buf + buflen, len, readf);
 				if (feof(readf))
 					break;
@@ -2931,7 +2931,7 @@ static enum agi_result run_agi(struct ast_channel *chan, char *request, AGI *agi
 				buflen = strlen(buf);
 				if (buflen && buf[buflen - 1] == '\n')
 					break;
-				len -= buflen;
+				len = sizeof(buf) - buflen;
 				if (agidebug)
 					ast_verbose( "AGI Rx << temp buffer %s - errno %s\n", buf, strerror(errno));
 			}