Fix unnecessary call to __strncpy_chk2

If "n" is smaller than the size of "src", then we'll
never read off the end of src. It makes no sense to call
__strncpy_chk2 in those circumstances.

For example, consider the following code:

int main() {
  char src[10];
  char dst[5];
  memcpy(src, "0123456789", sizeof(src));
  strncpy(dst, src, sizeof(dst));
  dst[4] = '\0';
  printf("%s\n", dst);
  return 0;
}

In this code, it's clear that the strncpy will never read off
the end of src.

Change-Id: I9cf58857a0c5216b4576d21d3c1625e2913ccc03

libc/include/string.h

@@ -135,6 +135,10 @@ char* strncpy(char* __restrict dest, const char* __restrict src, size_t n) {
         return __builtin___strncpy_chk(dest, src, n, bos_dest);
     }
 
+    if (__builtin_constant_p(n) && (n <= bos_src)) {
+        return __builtin___strncpy_chk(dest, src, n, bos_dest);
+    }
+
     size_t slen = __builtin_strlen(src);
     if (__builtin_constant_p(slen)) {
         return __builtin___strncpy_chk(dest, src, n, bos_dest);