[lldb] Fixed the error unable to launch a GDB server in API tests (#98833)

Summary:
TestPlatformLaunchGDBServer.py runs `ldb-server` w/o parameters
`--min-gdbserver-port`, `--max-gdbserver-port` or `--gdbserver-port`. So
`gdbserver_portmap` is empty and
`gdbserver_portmap.GetNextAvailablePort()` will return 0. Do not call
`portmap_for_child.AllowPort(0)` in this case. Otherwise
`portmap_for_child.GetNextAvailablePort()` will allocate and never free
the port 0 and next call `portmap_for_child.GetNextAvailablePort()` will
fail.

Added few asserts in `GDBRemoteCommunicationServerPlatform::PortMap` to
avoid such issue in the future.

This patch fixes a bug added in #88845. The behaviour is very close to
#97537 w/o parameters `--min-gdbserver-port`, `--max-gdbserver-port` and
`--gdbserver-port`.

Test Plan: 

Reviewers: 

Subscribers: 

Tasks: 

Tags: 


Differential Revision: https://phabricator.intern.facebook.com/D60250877

Author: slydiman

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationServerPlatform.cpp

@@ -46,11 +46,13 @@ using namespace lldb_private;
 
 GDBRemoteCommunicationServerPlatform::PortMap::PortMap(uint16_t min_port,
                                                        uint16_t max_port) {
+  assert(min_port);
   for (; min_port < max_port; ++min_port)
     m_port_map[min_port] = LLDB_INVALID_PROCESS_ID;
 }
 
 void GDBRemoteCommunicationServerPlatform::PortMap::AllowPort(uint16_t port) {
+  assert(port);
   // Do not modify existing mappings
   m_port_map.insert({port, LLDB_INVALID_PROCESS_ID});
 }

lldb/tools/lldb-server/lldb-platform.cpp

@@ -313,9 +313,11 @@ int main_platform(int argc, char *argv[]) {
       GDBRemoteCommunicationServerPlatform::PortMap portmap_for_child;
       llvm::Expected<uint16_t> available_port =
           gdbserver_portmap.GetNextAvailablePort();
-      if (available_port)
-        portmap_for_child.AllowPort(*available_port);
-      else {
+      if (available_port) {
+        // GetNextAvailablePort() may return 0 if gdbserver_portmap is empty.
+        if (*available_port)
+          portmap_for_child.AllowPort(*available_port);
+      } else {
         llvm::consumeError(available_port.takeError());
         fprintf(stderr,
                 "no available gdbserver port for connection - dropping...\n");
@@ -352,7 +354,7 @@ int main_platform(int argc, char *argv[]) {
     if (platform.IsConnected()) {
       if (inferior_arguments.GetArgumentCount() > 0) {
         lldb::pid_t pid = LLDB_INVALID_PROCESS_ID;
-        std::optional<uint16_t> port = 0;
+        std::optional<uint16_t> port;
         std::string socket_name;
         Status error = platform.LaunchGDBServer(inferior_arguments,
                                                 "", // hostname