ld2string should fail if string contains \0 in the middle

This bug affected RM_StringToLongDouble and HINCRBYFLOAT.
I added tests for both cases.

Main changes:
1. Fixed string2ld to fail if string contains \0 in the middle
2. Use string2ld in getLongDoubleFromObject - No point of
   having duplicated code here

The two changes above broke RM_SaveLongDouble/RM_LoadLongDouble
because the long double string was saved with length+1 (An innocent
mistake, but it's actually a bug - The length passed to
RM_SaveLongDouble should not include the last \0).
