Add a test for strncat

include/string.h

@@ -151,13 +151,12 @@ __access (read_only, 2, 3)
 _FORTIFY_FN(strncat) char *strncat(char *__d, const char *__s, size_t __n)
 {
 	size_t __b = __bos(__d, 0);
-	size_t __sl, __dl;
 
 	if (__n > __b) {
-		__sl = strlen(__s);
-		__dl = strlen(__d);
+		size_t __sl = strlen(__s);
 		if (__sl > __n)
 			__sl = __n;
+		size_t __dl = strlen(__d);
 		if (__sl + __dl + 1 > __b)
 			__builtin_trap();
 	}

tests/Makefile

@@ -19,6 +19,7 @@ TARGETS=test_memcpy_static_write     \
 	test_strcpy_overwrite_over   \
 	test_strcpy_overwrite_under  \
 	test_strcpy_static_write     \
+	test_strncat_static_write    \
 	test_strncpy_overwrite_over  \
 	test_strncpy_overwrite_under \
 	test_strncpy_static_write    \

tests/test_strncat_static_write.c

@@ -0,0 +1,16 @@
+#include "common.h"
+
+#include <string.h>
+
+int main(int argc, char** argv) {
+  char buffer[8] = {0};
+  strncat(buffer, "1234567", 5);
+  puts(buffer);
+
+  CHK_FAIL_START
+  strncat(buffer, "1234567890", 10);
+  CHK_FAIL_END
+
+  puts(buffer);
+  return ret;
+}