Fixes heap buffer overflow in ares_mkquery.

If name ends with "\.", ares_mkquery allocates a query buffer that is
one byte too small.

https://crbug.com/649040

--- c-ares-1.7.5/ares_mkquery.c
+++ c-ares-1.7.5/ares_mkquery.c
@@ -96,20 +96,22 @@ int ares_mkquery(const char *name, int dnsclass, int type, unsigned short id,
   *buflen = 0;
   *buf = NULL;
 
-  /* Compute the length of the encoded name so we can check buflen.
-   * Start counting at 1 for the zero-length label at the end. */
+  /* Compute the length of the encoded name so we can allocate the buffer.
+   * Start counting at 1 to account for the first label's length field. */
   len = 1;
   for (p = name; *p; p++)
     {
+      /* The length field of the last label is accounted for below. */
+      if (*p == '.' && *(p + 1) == 0)
+        break;
       if (*p == '\\' && *(p + 1) != 0)
         p++;
       len++;
     }
-  /* If there are n periods in the name, there are n + 1 labels, and
-   * thus n + 1 length fields, unless the name is empty or ends with a
-   * period.  So add 1 unless name is empty or ends with a period.
-   */
-  if (*name && *(p - 1) != '.')
+
+  /* Account for the length field of the last label. If the name is "." or
+   * empty, the last label is the first label, which was already counted. */
+  if (*name && strcmp(name, ".") != 0)
     len++;
 
   /* Immediately reject names that are longer than the maximum of 255