FIX: index out-of-bounds error when tab stop text is multi-line.

I had this when a ${1:`cmd`} substitution unexpectedly returned multiple lines:

Error detected while processing function TriggerSnippetAfterExpand..TriggerSnippet..snipMate#expandSnip..<SNR>480_BuildTabStops:
line   12:
E684: list index out of range: 20
Error detected while processing function TriggerSnippetAfterExpand..TriggerSnippet..snipMate#expandSnip..<SNR>480_BuildTabStops:
line   12:
E15: Invalid expression: a:indents[tabStopLnum] + len(matchstr(withoutOthers, '.*\(\n\|^\)\zs.*\ze${'.i.'\D'))

As snipMate does not support tabstop text spanning multiple lines (i.e. containing newline characters), remove them from the tap stop; this means that only the first line will be editable.