Fix "Dereference of null pointer" compiler warning.

This CL fixes the warning:
"frameworks/av/media/libstagefright/codecs/aacenc/src/dyn_bits.c:197:35
warning: Array access (from variable 'sideInfoTab') resutls in a null
pointer dereference"

There's a switch statement that initializes 'sideInfoTab', and it does
not have a default case.   The listed cases seem to cover all the valid
types for the switch variable, but since the declared type is basically
'int', it's possible for it to contain some value not covered by the
other cases, hit the non-existant default case, which leaves sideInfoTab
with its initial NULL value, later being dereferenced.  This CL fixes
this by adding a default case to the switch statement that complains if
it gets an invalid value.

Bug: None
Test: Compiles without the warning.
Change-Id: I1581b3337b6331a694774da27072dd39992e6218