Deal more gracefully with 0-size windows as suspected in issue 6005

sources/LineBlock.m

@@ -645,8 +645,9 @@ int OffsetOfWrappedLine(screen_char_t* p, int n, int length, int width, BOOL may
     return NULL;
 }
-- (int) getNumLinesWithWrapWidth: (int) width
-{
+- (int)getNumLinesWithWrapWidth:(int)width {
+    ITBetaAssert(width > 0, @"Bogus value of width: %d", width);
+    
     if (width == cached_numlines_width) {
         return cached_numlines;
     }

sources/PTYSession.m

@@ -1490,6 +1490,14 @@ ITERM_WEAKLY_REFERENCEABLE
 }
 - (void)setSize:(VT100GridSize)size {
+    ITBetaAssert(size.width > 0, @"Nonpositive width %d", size.width);
+    ITBetaAssert(size.height > 0, @"Nonpositive height %d", size.height);
+    if (size.width <= 0) {
+        size.width = 1;
+    }
+    if (size.height <= 0) {
+        size.height = 1;
+    }
     self.lastResize = [NSDate timeIntervalSinceReferenceDate];
     DLog(@"Set session %@ to %@", self, VT100GridSizeDescription(size));
     [_screen setSize:size];