Fix OPEN COM from locking up inside gfs_open_com_syntax()

[a740g]

This PR fixes an OPEN COM statement from locking up inside gfs_open_com_syntax().

For example, the following code crashes with v3.9.0, v3.9.1 and locks up on later versions.

The issue was first reported here: https://qb64phoenix.com/forum/showthread.php?tid=2376&pid=22478#pid22478

ON ERROR GOTO Handler
OPEN "COM4:74880,N,8,1,BIN,CS0,DS0,RB8192" FOR RANDOM AS #99
END

Handler:
PRINT "Error:"; ERR
RESUME NEXT

With the changes in the PR, this no longer happens. The changes mostly consist of re-formatting the code and improving readability. But most importantly, it now returns 1 if it can completely parse the string.