Improve str_truncate

Check start_column <  wcols and issue error
Reduce output length by 1 to take care a
situation where output_length maybe provided
by strlen which excludes the null terminator
and sizeof which may or may count the terminator.

src/gui.c

@@ -8257,16 +8257,22 @@ char* str_truncate( int wcols, int start_column, const char* input, char* output
      */
 
     int length, idx = 0;
-
+    if( start_column < wcols )
+    {
         length = wcols - start_column - 1;
         idx = 0;
-    while( idx < output_length && idx < length )
+        while( idx < output_length - 1 && idx < length && input[idx] != 0 )
         {
             output[idx] = input[idx];
             idx++;
         }
         /* terminate the string */
         output[idx] = 0;
+    }
+    else
+    {
+        strncpy( output, "Error:start_column>=wcols", output_length - 1 );
+    }
 
     return output;
 }