Audit all the LASSERT calls, and try to do something sensible at

Audit all the LASSERT calls, and try to do something sensible at
each failure.

There are several places I was not sure what to do. These are marked
by comments beginning "LASSERT:" so they can be found easily. At the
moment, they are at:

Author.cpp:105: // LASSERT: What should we do here?
Author.cpp:121: // LASSERT: What should we do here?
Buffer.cpp:4525:        // LASSERT: Is it safe to continue here, or should we just return?
Cursor.cpp:345:         // LASSERT: Is it safe to continue here, or should we return?
Cursor.cpp:403:         // LASSERT: Is it safe to continue here, or should we return?
Cursor.cpp:1143:                // LASSERT: There have been several bugs around this code, that seem
CursorSlice.cpp:83:     // LASSERT: This should only ever be called from an InsetMath.
CursorSlice.cpp:92:     // LASSERT: This should only ever be called from an InsetMath.
LayoutFile.cpp:303:                     // LASSERT: Why would this fail?
Text.cpp:995:           // LASSERT: Is it safe to continue here?