clamp instead of assert when running out of room during reflow

indra/llui/lltextbase.cpp

@@ -1137,6 +1137,10 @@ void LLTextBase::reflow(S32 start_index)
 			// grow line height as necessary based on reported height of this segment
 			line_height = llmax(line_height, segment_height);
 			remaining_pixels -= segment_width;
+			if (remaining_pixels < 0)
+			{
+				remaining_pixels = 0;
+			}
 
 			// getNumChars() and getDimensions() should return consistent results
 			llassert_always(remaining_pixels >= 0);