utils: Fix TOCTOU in get_pid_exe()

This fixes a classic TOCTOU. The problem is that if `pid` exits between
the exists() check and read_symlink() we will eat a ENOENT exception.
Fix by checking result of read_symlink() operation itself.

Discovered in a flakey test run in another PR:

[  FAILED  ] builtin.func_uprobe_elf_symtable
	Command: /home/runner/work/bpftrace/bpftrace/build-ci/src/bpftrace -e 'uprobe:./testprogs/uprobe_symres_exited_process:test { print(func); exit(); }'
	Unclean exit code: -6
	Output: Attaching 1 probe...\n__BPFTRACE_NOTIFY_PROBES_ATTACHED\nterminate called after throwing an instance of 'std::filesystem::__cxx11::filesystem_error'\n  what():  filesystem error: read_symlink: No such file or directory [/proc/13255/exe]\n

src/utils.cpp

@@ -274,11 +274,18 @@ std::string get_pid_exe(const std::string &pid)
   proc_path /= pid;
   proc_path /= "exe";
 
-  if (!std_filesystem::exists(proc_path, ec) ||
-      !std_filesystem::is_symlink(proc_path, ec))
-    return "";
-
-  return std_filesystem::read_symlink(proc_path).string();
+  try
+  {
+    return std_filesystem::read_symlink(proc_path).string();
+  }
+  catch (const std_filesystem::filesystem_error &e)
+  {
+    auto err = e.code().value();
+    if (err == ENOENT || err == EINVAL)
+      return {};
+    else
+      throw e;
+  }
 }
 
 std::string get_pid_exe(pid_t pid)