Fix `ledger-string-to-number' for decimal-comma

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Hi John,

Small bug, caused by ill-formatted elisp. This is a small patch, so i
think making a fork & opening a Pull Request is a bit too much. I'll go
this route if I make bigger contributions in the future.

From de1fb19ded5300a227d58e3d3094928b211db09b Mon Sep 17 00:00:00 2001
From: Moritz Ulrich <moritz@tarn-vedra.de>
Date: Sat, 28 Dec 2013 22:37:54 +0100
Subject: [PATCH] ledger-string-to-number: Fix nil return value with
 decimal-comma.

Unfavorable formatting hid this bug pretty well. In case of true-ish
`decimal-comma' or "decimal-comma' in `ledger-environment-alist', the
ill-formatted `if' (and the whole function) returns nil.

Signed-off-by: Moritz Ulrich <moritz@tarn-vedra.de>