base: Use the correct string size in BaseRemoteGDB::cmd_query_var.

The size of the command should be len, the length of the command string,
not len - 1. Looking at query strings shows that they were previously
being truncated by one character.

Change-Id: I283891eadafaa07d12453f085e8a106d59a4f889
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44026
Reviewed-by: Boris Shingarov <shingarov@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/remote_gdb.cc

@@ -922,7 +922,7 @@ BaseRemoteGDB::cmd_mem_w(GdbCommand::Context &ctx)
 bool
 BaseRemoteGDB::cmd_query_var(GdbCommand::Context &ctx)
 {
-    std::string s(ctx.data, ctx.len - 1);
+    std::string s(ctx.data, ctx.len);
     std::string xfer_read_prefix = "Xfer:features:read:";
     if (s.rfind("Supported:", 0) == 0) {
         std::ostringstream oss;