Undefined behavior in utf8_decode()

Hello,

there is an issue with undefined behavior in lutf8lib.c because shift 
exponent may be too large for 32-bit type 'l_uint32':

lutf8lib.c:67:34: runtime error: shift exponent 35 is too large for 32-bit 
type 'l_uint32' (aka 'unsigned int')
    #0 0x56308fd6face in utf8_decode 
/src/testdir/build/lua-master/source/lutf8lib.c:67:34
    #1 0x56308fd6f4fd in utflen 
/src/testdir/build/lua-master/source/lutf8lib.c:99:22
    #2 0x56308fd05fe9 in precallC 
/src/testdir/build/lua-master/source/ldo.c:663:7
    #3 0x56308fd0687c in luaD_precall 
/src/testdir/build/lua-master/source/ldo.c
    #4 0x56308fd40413 in luaV_execute 
/src/testdir/build/lua-master/source/lvm.c:1729:22
    #5 0x56308fd06ce3 in ccall 
/src/testdir/build/lua-master/source/ldo.c:774:5
    #6 0x56308fd03b69 in luaD_rawrunprotected 
/src/testdir/build/lua-master/source/ldo.c:166:3
    #7 0x56308fd07cd8 in luaD_pcall 
/src/testdir/build/lua-master/source/ldo.c:1096:12
    #8 0x56308fcfaaa0 in lua_pcallk 
/src/testdir/build/lua-master/source/lapi.c:1097:14
    #9 0x56308fcf1435 in LLVMFuzzerTestOneInput 
/src/testdir/tests/capi/luaL_loadbuffer_test.c:34:3

Steps to reproduce:

make MYCFLAGS=-fsanitize=undefined MYLDFLAGS=-fsanitize=undefined
echo "dXRmOC5sZW4n2b0B/4+Pj4+Pj48AACc=" | base64 --decode | ./lua -

Lua version: c6b48482

Sergey

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