misc/string.c: fix one byte ahead access in dup_string

In dup_string in misc/string.c , even if the destination
buffer is allocated with n_char bytes,
the source buffer `str` can be accessed up to only n_char bytes.

src/misc/string.c

@@ -78,7 +78,7 @@ char *dup_string(const char *str, size_t n_char)
     char *p = TMALLOC(char, n_char + 1);
 
     if (p != NULL) {
-        (void) memcpy(p, str, n_char + 1);
+        (void) memcpy(p, str, n_char);
         p[n_char] = '\0';
     }
     return p;