ShellPkg/CommandLib: avoid NULL derefence and memory leak

ShellPkg/CommandLib: avoid NULL derefence and memory leak

Since TianoCore EDK2 commit d65f2cea36d1 ("ShellPkg/CommandLib: Locate
proper UnicodeCollation instance") in edk2 the UEFI Shell crashes if EFI
variable PlatformLang is not defined due to dereferencing gUnicodeCollation
gUnicodeCollation (= NULL) in ShellCommandRegisterCommandName().

Furthermore CommandInit() is leaking PlatformLang if
gUnicodeCollation != NULL.

Close the memory leak and use the first UnicodeCollation instance if
PlatfomLang is not defined.

Fixes: d65f2cea36d1 ("ShellPkg/CommandLib: Locate proper UnicodeCollation
instance")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Zhichao Gao <zhichao.gao@intel.com>
Reviewed-by: Jaben Carsey <jaben.carsey@intel.com>