ssh: use stack scratch buffer for seeks

- instead of data->state.buffer

Closes #12794

lib/vssh/libssh.c

@@ -1308,13 +1308,14 @@ static CURLcode myssh_statemach_act(struct Curl_easy *data, bool *block)
           }
           /* seekerr == CURL_SEEKFUNC_CANTSEEK (can't seek to offset) */
           do {
+            char scratch[4*1024];
             size_t readthisamountnow =
-              (data->state.resume_from - passed > data->set.buffer_size) ?
-              (size_t)data->set.buffer_size :
-              curlx_sotouz(data->state.resume_from - passed);
+              (data->state.resume_from - passed >
+                (curl_off_t)sizeof(scratch)) ?
+              sizeof(scratch) : curlx_sotouz(data->state.resume_from - passed);
 
             size_t actuallyread =
-              data->state.fread_func(data->state.buffer, 1,
+              data->state.fread_func(scratch, 1,
                                      readthisamountnow, data->state.in);
 
             passed += actuallyread;

lib/vssh/libssh2.c

@@ -2158,14 +2158,15 @@ static CURLcode ssh_statemach_act(struct Curl_easy *data, bool *block)
           }
           /* seekerr == CURL_SEEKFUNC_CANTSEEK (can't seek to offset) */
           do {
+            char scratch[4*1024];
             size_t readthisamountnow =
-              (data->state.resume_from - passed > data->set.buffer_size) ?
-              (size_t)data->set.buffer_size :
-              curlx_sotouz(data->state.resume_from - passed);
+              (data->state.resume_from - passed >
+                (curl_off_t)sizeof(scratch)) ?
+              sizeof(scratch) : curlx_sotouz(data->state.resume_from - passed);
 
             size_t actuallyread;
             Curl_set_in_callback(data, true);
-            actuallyread = data->state.fread_func(data->state.buffer, 1,
+            actuallyread = data->state.fread_func(scratch, 1,
                                                   readthisamountnow,
                                                   data->state.in);
             Curl_set_in_callback(data, false);

lib/vssh/wolfssh.c

@@ -641,14 +641,15 @@ static CURLcode wssh_statemach_act(struct Curl_easy *data, bool *block)
           }
           /* seekerr == CURL_SEEKFUNC_CANTSEEK (can't seek to offset) */
           do {
+            char scratch[4*1024];
             size_t readthisamountnow =
-              (data->state.resume_from - passed > data->set.buffer_size) ?
-              (size_t)data->set.buffer_size :
-              curlx_sotouz(data->state.resume_from - passed);
+              (data->state.resume_from - passed >
+                (curl_off_t)sizeof(scratch)) ?
+              sizeof(scratch) : curlx_sotouz(data->state.resume_from - passed);
 
             size_t actuallyread;
             Curl_set_in_callback(data, true);
-            actuallyread = data->state.fread_func(data->state.buffer, 1,
+            actuallyread = data->state.fread_func(scratch, 1,
                                                   readthisamountnow,
                                                   data->state.in);
             Curl_set_in_callback(data, false);