ext/openssl/ossl.h: Remove a variable that is used only in assert

It produces "unused variable" warnings in NDEBUG mode

ext/openssl/ossl.h

@@ -88,9 +88,8 @@ VALUE ossl_buf2str(char *buf, int len);
 VALUE ossl_str_new(const char *, long, int *);
 #define ossl_str_adjust(str, p) \
 do{\
-    long len = RSTRING_LEN(str);\
     long newlen = (long)((p) - (unsigned char*)RSTRING_PTR(str));\
-    assert(newlen <= len);\
+    assert(newlen <= RSTRING_LEN(str));\
     rb_str_set_len((str), newlen);\
 }while(0)
 /*