* src/type1/t1afm.c (T1_Read_Metrics): Validate ascender and descender.

The ascender and descender are optional in the AFM specifications.
They could be omitted or even set to zero, e.g., in the current release
of URW++ base 35 fonts.

src/type1/t1afm.c

@@ -296,9 +296,13 @@
       t1_face->bbox.xMax = ( fi->FontBBox.xMax + 0xFFFF ) >> 16;
       t1_face->bbox.yMax = ( fi->FontBBox.yMax + 0xFFFF ) >> 16;
 
-      /* no `U' suffix here to 0x8000! */
-      t1_face->ascender  = (FT_Short)( ( fi->Ascender  + 0x8000 ) >> 16 );
-      t1_face->descender = (FT_Short)( ( fi->Descender + 0x8000 ) >> 16 );
+      /* ascender and descender are optional and could both be zero */
+      if ( fi->Ascender || fi->Descender )
+      {  
+        /* no `U' suffix here to 0x8000! */
+        t1_face->ascender  = (FT_Short)( ( fi->Ascender  + 0x8000 ) >> 16 );
+        t1_face->descender = (FT_Short)( ( fi->Descender + 0x8000 ) >> 16 );
+      }
 
       if ( fi->NumKernPair )
       {