Resolves: tdf#118073 one leading 0 is significant for 0.

Regression from

    commit 9a6527a9
    CommitDate: Mon Jun 27 21:57:52 2016 +0200

        stringToDouble() do not parse separator without digit as 0.0

Change-Id: I9d90aedc324ef0938297224297d89817e3fd1e90
Reviewed-on: https://gerrit.libreoffice.org/56028Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Jenkins

sal/rtl/math.cxx

@@ -817,7 +817,7 @@ inline double stringToDouble(CharT const * pBegin, CharT const * pEnd,
             ++p;
         }
 
-        CharT const * pFirstSignificant = p;
+        CharT const * pFirstSignificant = ((p > pBegin && *(p-1) == CharT('0')) ? p-1 : p);
         long nValExp = 0;       // carry along exponent of mantissa
 
         // integer part of mantissa