winmm: Avoid casting return value of Heap(Re)Alloc calls.

[Dan Kegel]

Doesn't build here:

ccache gcc -c -I. -I. -I../../include -I../../include  -D__WINESRC__
-D_WINMM_ -D_REENTRANT -fPIC -Wall -pipe -fno-strict-aliasing
-Wdeclaration-after-statement -Wempty-body -Wstrict-prototypes
-Wtype-limits -Wunused-but-set-parameter -Wwrite-strings
-fno-omit-frame-pointer -Wpointer-arith -Wlogical-op
-I/usr/include/freetype2  -g -O0 -Werror -Wno-unused-but-set-variable
-Wno-unused-but-set-parameter  -o lolvldrv.o lolvldrv.c
lolvldrv.c: In function 'MMDRV_InitPerType':
lolvldrv.c:406:13: error: pointer of type 'void *' used in arithmetic
[-Werror=pointer-arith]
lolvldrv.c:411:13: error: pointer of type 'void *' used in arithmetic
[-Werror=pointer-arith]
cc1: all warnings being treated as errors
make[1]: Leaving directory
`/home/bob/wineslave.dir/sandbox/slave/runtests-default/build/dlls/winmm'
make[1]: *** [lolvldrv.o] Error 1
make: *** [dlls/winmm] Error 2

http://buildbot.kegel.com/builders/runtests-default/builds/874