Initialize variable in runtime/softfloat.c to avoid RHEL8 -Werror issue

Initialize variable in runtime/softfloat.c to avoid RHEL8 -Werror issue

Make sure that the variable is initialized to something to avoid the
following error when running the testsuite on RHEL8:

attempting command stap -p4 floatingpoint.stp -c "stap --benchmark-sdt"
OUT In file included from /tmp/stapBRN9va/stap_825f154f474bfd5b2080a28426f65178_4743_src.c:37:
/usr/share/systemtap/runtime/softfloat.c: In function 'softfloat_shiftRightJamM':
/usr/share/systemtap/runtime/softfloat.c:132:34: error: 'ptr' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     uint32_t wordJam, wordDist, *ptr;
                                  ^~~
cc1: all warnings being treated as errors
make[3]: *** [scripts/Makefile.build:315: /tmp/stapBRN9va/stap_825f154f474bfd5b2080a28426f65178_4743_src.o] Error 1
make[2]: *** [Makefile:1544: _module_/tmp/stapBRN9va] Error 2
WARNING: kbuild exited with status: 2
Pass 4: compilation failed.  [man error::pass4]
child process exited abnormally
RC 1
FAIL: systemtap.examples/general/floatingpoint build