text-freetype2: Fix crash after reaching buffer size

The texbuf has a fixed size that won't expand. When a lot of multi-byte
characters has arrived, it overflow and src_glyph becomes NULL.

(cherry picked from commit 4fb2bc38de4170d85b6327bd9e62f9f658007e86)

plugins/text-freetype2/text-functionality.c

@@ -140,7 +140,8 @@ void set_up_vertex_buffer(struct ft2_source *srcdata)
 	next_char:;
 		glyph_index =
 			FT_Get_Char_Index(srcdata->font_face, srcdata->text[i]);
-		word_width += src_glyph->xadv;
+		if (src_glyph)
+			word_width += src_glyph->xadv;
 	eos_skip:;
 	}