kbuild: check return value of asprintf()

Check return value of asprintf() in docsect() and exit if error
occurs. This removes following warning:

  HOSTCC  scripts/basic/docproc
scripts/basic/docproc.c: In function ‘docsect’:
scripts/basic/docproc.c:336: warning: ignoring return value of ‘asprintf’,
				declared with attribute warn_unused_result
Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/basic/docproc.c

@@ -333,7 +333,10 @@ static void docsect(char *filename, char *line)
 		if (*s == '\n')
 			*s = '\0';
 
-	asprintf(&s, "DOC: %s", line);
+	if (asprintf(&s, "DOC: %s", line) < 0) {
+		perror("asprintf");
+		exit(1);
+	}
 	consume_symbol(s);
 	free(s);