fix clang -Wlogical-op warnings

Commit amended by @perlpunk after suggestion from @tlsa

src/scanner.c

@@ -2860,7 +2860,7 @@
 
     if (!CACHE(parser, 1)) goto error;
 
-    while ((int)parser->mark.column == indent && !IS_Z(parser->buffer))
+    while ((int)parser->mark.column == indent && !(IS_Z(parser->buffer)))
     {
         /*
          * We are at the beginning of a non-empty line.

src/yaml_private.h

@@ -178,7 +178,7 @@
  * Check the current octet in the buffer.
  */
 
-#define CHECK(string,octet) CHECK_AT((string),(octet),0)
+#define CHECK(string,octet) (CHECK_AT((string),(octet),0))
 
 /*
  * Check if the character at the specified position is an alphabetical