SSI: avoid potential buffer overflow.

When "-" follows a parameter of maximum length, a single byte buffer
overflow happens, since the error branch does not check parameter length.
Fix is to avoid saving "-" to the parameter key, and instead use an error
message with "-" explicitly written.  The message is mostly identical to
one used in similar cases in the preequal state.

Reported by Patrick Wollgast.

src/http/modules/ngx_http_ssi_filter_module.c

diff --git a/src/http/modules/ngx_http_ssi_filter_module.c b/src/http/modules/ngx_http_ssi_filter_module.c
index d608df9..6737965 100644
--- a/src/http/modules/ngx_http_ssi_filter_module.c
+++ b/src/http/modules/ngx_http_ssi_filter_module.c
@@ -1254,9 +1254,9 @@
             case '-':
                 state = ssi_error_end0_state;
 
-                ctx->param->key.data[ctx->param->key.len++] = ch;
                 ngx_log_error(NGX_LOG_ERR, r->connection->log, 0,
-                              "invalid \"%V\" parameter in \"%V\" SSI command",
+                              "unexpected \"-\" symbol after \"%V\" "
+                              "parameter in \"%V\" SSI command",
                               &ctx->param->key, &ctx->command);
                 break;