icu: fix NULL dereference in `unum_clone`

If `dynamic_cast` fails, then NULL is returned. Even thought
assertion is set, we cannot rely on it, as we don't use debug
version of icu. Let's check if `rbnf` variable is not NULL
explicitly.

If it somehow turned out to be NULL, then memory allocation
error will be thrown.

Closes tarantool/security#61

NO_CHANGELOG=<security fix>
NO_DOC=<security fix>
NO_TEST=<third-party security fix>

static-build/cmake/AddDependencyProjects.cmake

@@ -116,6 +116,7 @@ ExternalProject_Add(icu
         ${CMAKE_COMMAND} -E copy_if_different <BINARY_DIR>/uconfig.h <INSTALL_DIR>/include/unicode/uconfig.h
     PATCH_COMMAND patch -d <SOURCE_DIR> -p1 -i "${PATCHES_DIR}/icu-tarantool-security-45.patch"
     COMMAND       patch -d <SOURCE_DIR> -p1 -i "${PATCHES_DIR}/icu-tarantool-security-59.patch"
+    COMMAND       patch -d <SOURCE_DIR> -p1 -i "${PATCHES_DIR}/icu-tarantool-security-61.patch"
 )
 set(TARANTOOL_DEPENDS icu ${TARANTOOL_DEPENDS})
 

static-build/patches/icu-tarantool-security-61.patch

+diff --git a/source/i18n/unum.cpp b/source/i18n/unum.cpp
+index 7043f7a..0f70c5c 100644
+--- a/source/i18n/unum.cpp
++++ b/source/i18n/unum.cpp
+@@ -164,7 +164,9 @@ unum_clone(const UNumberFormat *fmt,
+     } else {
+         const RuleBasedNumberFormat* rbnf = dynamic_cast<const RuleBasedNumberFormat*>(nf);
+         U_ASSERT(rbnf != NULL);
+-        res = rbnf->clone();
++        if (rbnf != NULL) {
++            res = rbnf->clone();
++        }
+     }
+ 
+     if(res == 0) {