Fixes undefined behavior warnings from clang.

  external/libgsm/src/short_term.c:68:2: warning: shifting a negative signed value is undefined [-Wshift-negative-value]
        STEP(  -1144,   -4,  29708 );
        ^      ~~~~~

Bug: http://b/26694729
Test: builds
Change-Id: I65063e1f4ba71c6231306baa7ae8c8fded7d4c89