Fix bad null check in pal_dsa.c

Fix bad null check in pal_dsa.c

The code calls DSA_new() and then, only after assigning the value from
DSA_new to a pointer, tries to check that the pointer was not null.
Assigning to a null pointer leads to undefined behaviour and a compiler
can assume that assignment-to-a-null-pointer wont happen. Subsequently,
the compiler can remove the entire check because the pointer can't be
null.

Fix that by:

1. Moving the check for whether the pointer is null before dereferncing
   the pointer.

2. Checking the return value of DSA_new() spearately after it has been
   called.

Commit migrated from https://github.com/dotnet/corefx/commit/e7fd1669eafafd16de85cd90b558d54abb3a3343