[LyX/master] Fix formatFPnumber with negative doubles.

[Juergen Spitzmueller]

commit c9d9309c1ecffa218dee04ce4f7991ed4fc0c9bb
Author: Juergen Spitzmueller <sp...@lyx.org>
Date:   Fri Jun 26 15:49:44 2015 +0200

    Fix formatFPnumber with negative doubles.

diff --git a/src/support/lstrings.cpp b/src/support/lstrings.cpp
index c77c986..6faf1a4 100644
--- a/src/support/lstrings.cpp
+++ b/src/support/lstrings.cpp
@@ -1407,6 +1407,10 @@ std::string formatFPNumber(double x)
        os << std::fixed;
        // Prevent outputs of 23.4200000000000017 but output small numbers
        // with at least 6 significant digits.
+       bool const neg = x < 0;
+       // Treat all doubles as positive for the formatting
+       if (neg)
+               x = -x;
        double const logarithm = log10(x);
        os << std::setprecision(max(6 - static_cast<int>(round(logarithm)), 0)) 
<< x;
        string result = os.str();
@@ -1415,6 +1419,9 @@ std::string formatFPNumber(double x)
                if (result[result.length()-1] == '.')
                        result = rtrim(result, ".");
        }
+       // Re-add negativity
+       if (neg)
+               result = "-" + result;
        return result;
 }