Fix first/last image indexing bug

Fixes #301

doc/imv.5.txt

@@ -134,7 +134,7 @@ or a modifier is used it must be wrapped in '<' and '>', for example:
 '<Ctrl+q>'.
 
 Multiple keys in succession can be specified by listing them in order:
-'gg = goto 0' will bind two presses of the 'g' key to jump to the first
+'gg = goto 1' will bind two presses of the 'g' key to jump to the first
 image, and '<Ctrl+a>p = exec echo hi' will bind the key sequence of 'Ctrl+a'
 followed by 'p' to executing the shell command 'echo hi'.
 

files/imv_config

@@ -21,7 +21,7 @@ y = exec echo working!
 <bracketleft> = prev
 <Right> = next
 <bracketright> = next
-gg = goto 0
+gg = goto 1
 <Shift+G> = goto -1
 
 # Panning

src/imv.c

@@ -1641,7 +1641,7 @@ static void command_goto(struct list *args, const char *argstr, void *data)
   }
 
   long int index = strtol(args->items[1], NULL, 10);
-  imv_navigator_select_abs(imv->navigator, index - 1);
+  imv_navigator_select_abs(imv->navigator, index > 0 ? index - 1 : index);
   imv_viewport_reset_transform(imv->view);
 
   imv->slideshow.elapsed = 0;