[base] Avoid undefined behaviour in lcd filtering code (#53727).

* src/base/ftlcdfil.c (ft_lcd_filter_fir, _ft_lcd_filter_legacy):
Ensure `height > 0'.

ChangeLog

@@ -1,3 +1,10 @@
+2018-04-24  Ben Wagner  <bungeman@google.com>
+
+	[base] Avoid undefined behaviour in lcd filtering code (#53727).
+
+	* src/base/ftlcdfil.c (ft_lcd_filter_fir, _ft_lcd_filter_legacy):
+	Ensure `height > 0'.
+
 2018-04-22  Werner Lemberg  <wl@gnu.org>
 
 	* src/base/ftoutln.c (FT_Outline_Decompose): Improve error tracing.

src/base/ftlcdfil.c

@@ -77,7 +77,7 @@
 
 
     /* take care of bitmap flow */
-    if ( pitch > 0 )
+    if ( pitch > 0 && height > 0 )
       origin += pitch * (FT_Int)( height - 1 );
 
     /* horizontal in-place FIR filter */
@@ -192,7 +192,7 @@
 
 
     /* take care of bitmap flow */
-    if ( pitch > 0 )
+    if ( pitch > 0 && height > 0 )
       origin += pitch * (FT_Int)( height - 1 );
 
     /* horizontal in-place intra-pixel filter */