Make sure TERM_PROGRAM points to a valid program

Visual Studio Code sets the $TERM_PROGRAM environment variable to `"vscode"` in its terminal pane,
but doesn't support opening new panes from the command line and there is no "vscode" binary.

Make sure the target binary exists before trying to launch it.

pwnlib/util/misc.py

@@ -297,7 +297,7 @@ def run_in_new_terminal(command, terminal=None, args=None, kill_at_exit=True, pr
         elif 'STY' in os.environ and which('screen'):
             terminal = 'screen'
             args     = ['-t','pwntools-gdb','bash','-c']
-        elif 'TERM_PROGRAM' in os.environ:
+        elif 'TERM_PROGRAM' in os.environ and which(os.environ['TERM_PROGRAM']):
             terminal = os.environ['TERM_PROGRAM']
             args     = []
         elif 'DISPLAY' in os.environ and which('x-terminal-emulator'):