Don't attempt to extract a URL from invalid coordinates (-1,-1)

[anthroid]

When the terminal context menu is opened from a session titlebar, the coordinates passed to menuAtCoord:(VT100GridCoord)coord are (-1,-1), of which the y-coordinate is used as an array index to extract a line from the terminal grid in order to extract a URL. This causes a crash due to a [-1] array index downstream in getLineAtIndex:(int)theIndex as the context menu is being built when left-clicking the session titlebar context menu button in all versions after 2534252 with the following message:

Performing @selector(openMenu:) from sender NSButton 0x600001e6c4d0
Assertion failed: (NO), function -[VT100Screen getLineAtIndex:withBuffer:],
file /Users/.../iTerm2/sources/VT100Screen.m, line 1804.

This PR adds a lower bounds check on the coordinates before attempting to retrieve the line and extract the URL.

[gnachman]

Thank you!