Change NULL to nullptr in gdb/infcmd.c and gdb/infrun.c

Change NULL to nullptr in gdb/infcmd.c and gdb/infrun.c

The GDB coding standard specifies that nullptr should be used instead of
NULL.  There are numerous uses of NULL and nullptr in files infcmd.c and
infrun.c.  This patch replaces the various uses of NULL with nullptr in
the source files.  The use of NULL in the comments was not changed.

The patch does not introduce any functional changes.

The patch has been tested on PowerPC and Intel X86_64 with no new unexpected
test failures, unresolved tests, new core files etc.