xpath BUGFIX print_expr_str undefined behavior (#2126)

clang 16 with ASAN and UBSAN flag this as UB because it's performing
pointer arithmetic on a NULL pointer, which is not allowed in C.
The test suite of libyang-cpp triggers this behavior:

 2/4 Test #1: test_context .....................***Failed    0.77 sec
 libyang[0]: Unexpected end-of-input. (path: Line number 1.)
 libyang[0]: Data model "invalid" not found in local searchdirs.
 libyang[0]: Loading "invalid" module failed.
 libyang[0]: Feature "nonexisting" not found in module "mod1".
 libyang[0]: Data model "doesnt-exist" not found in local searchdirs.
 libyang[0]: Loading "doesnt-exist" module failed.
 /home/ci/src/cesnet-gerrit-public/github/CESNET/libyang/src/xpath.c:239:24: runtime error: applying zero offset to null pointer
     #0 0x7f4054b4d168 in print_expr_str /home/ci/src/cesnet-gerrit-public/github/CESNET/libyang/src/xpath.c:239:24
     #1 0x7f4054b2ba16 in print_expr_struct_debug /home/ci/src/cesnet-gerrit-public/github/CESNET/libyang/src/xpath.c:276:9
     #2 0x7f4054b27209 in lyxp_expr_parse /home/ci/src/cesnet-gerrit-public/github/CESNET/libyang/src/xpath.c:3216:5
     #3 0x7f40543582ab in ly_path_parse /home/ci/src/cesnet-gerrit-public/github/CESNET/libyang/src/path.c:339:5
     #4 0x7f405441c8d4 in lyd_find_path /home/ci/src/cesnet-gerrit-public/github/CESNET/libyang/src/tree_data.c:3028:11
     #5 0x7f4055751848 in libyang::DataNode::findPath(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char>> const&, libyang::OutputNodes) const /home/ci/src/cesnet-gerrit-public/CzechLight/libyang-cpp/src/DataNode.cpp:220:16
     #6 0x5424d5 in DOCTEST_ANON_FUNC_2() /home/ci/src/cesnet-gerrit-public/CzechLight/libyang-cpp/tests/context.cpp:346:9
     #7 0x5ffbf7 in doctest::Context::run() /home/ci/target/include/doctest/doctest.h:7007:21
     #8 0x60556d in main /home/ci/target/include/doctest/doctest.h:7085:71
     #9 0x7f4055048b49 in __libc_start_call_main /usr/src/debug/glibc-2.37-1.fc38.x86_64/csu/../sysdeps/nptl/libc_start_call_main.h:58:16
     #10 0x7f4055048c0a in __libc_start_main@GLIBC_2.2.5 /usr/src/debug/glibc-2.37-1.fc38.x86_64/csu/../csu/libc-start.c:360:3
     #11 0x4450f4 in _start (/home/ci/build/CzechLight/libyang-cpp/test_context+0x4450f4) (BuildId: b2cbc6b91e2c885110ddbf157ff7a19e41ed0306)

 SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /home/ci/src/cesnet-gerrit-public/github/CESNET/libyang/src/xpath.c:239:24 in

I'm getting this on Fedora 38 and on Arch, but for some reason I cannot
seem to hit the original issue on NixOS. Strange, but it's an UB anyway.

Fixes: 1696178 xpath UPDATE do not use memstream

src/xpath.c

@@ -236,7 +236,7 @@ print_expr_str(char **str, size_t *size, size_t *used, const char *format, ...)
     va_start(ap, format);
 
     /* try to append the string */
-    p = vsnprintf(*str + *used, *size - *used, format, ap);
+    p = vsnprintf(*str ? *str + *used : NULL, *size - *used, format, ap);
 
     if ((unsigned)p >= *size - *used) {
         /* realloc */