fix: Prevent crash in fteapot when rendering very narrow columns (#76)

Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Reviewed-on: #76

INSTALL

@@ -1,6 +1,14 @@
 The new build system uses CMake (http://www.cmake.org). It handles
 all platform differences.
 
+Once all prerequisites are in place, you should be able to build teapot via
+```
+mkdir build
+cd build
+cmake ..
+make
+```
+
 In addition to the information in the README.md file on prerequisites, here
 are some additional guidelines:
 

src/common/wk1.h

@@ -1,5 +1,6 @@
 #ifndef WK1_H
 #define WK1_H
+/* Functions for reading Lotus 1-2-3 file format. */
 
 #include "sheet.h"
 

src/common/xdr.c

@@ -1,5 +1,7 @@
 /* Notes */ /*{{{C}}}*//*{{{*/
 /*
+This file contains code to read the legacy binary XDR format for
+spreadsheet files, typically written in files with the .tp extension.
 
 xdr_enum() is unusable, because enum_t may have a different size than
 an enum.  The construction

src/common/xdr.h

@@ -1,5 +1,6 @@
 #ifndef XDR_H
 #define XDR_H
+/* Functions used for reading the legacy XDR binary file format */
 
 /* Thanks to curses. */
 #if 0

src/fteapot.fl

@@ -78,6 +78,12 @@ class TeapotTable {open : {public Fl_Table}}
     {draw_cell(TableContext context, int R, int C, int xx, int yy,
                int W, int H)} {open return_type void} {code
     {
+        // Fl_Table calls this function to draw each visible cell in the table.
+        // context specifies what part of the table is being drawn;
+        // R and C give the row and column of the cell in the table;
+        // xx and yy give the screen coordinates where the cell is to be drawn;
+        // and W and H give the width and height of the rectangle into which to
+        // draw.
         char s[1024];
         if (debug_level > 2)
             printf("DRAW: %i @%i,%i - (%i,%i) %ix%i\\n",
@@ -167,13 +173,14 @@ class TeapotTable {open : {public Fl_Table}}
 	    if (sc.italic) cellfont |= FL_ITALIC;
 	    fl_font(cellfont, 14);
 
-	    size_t len = printvalue(s, sizeof(s), 0, quote, sc.fform,
+	    size_t ulen = printvalue(s, sizeof(s), 0, quote, sc.fform,
 					sc.precision, cursheet, test);
-	    size_t nlen = len;
+	    ptrdiff_t len = ulen;
+	    ptrdiff_t nlen = len;
 	    int ww = 0, hh = 0;
 	    fl_measure(s, ww, hh, 0);
 	    while (ww > W-6) {
-		char *dc = mbspos(s+nlen-1, -1);
+		char *dc = mbspos(s+nlen, -1);
 		nlen = dc - s;
 	        s[nlen] = (char)0;
 		ww = 0;