vm_backtrace.c: add RB_GC_GUARD for name in location_format

`name` is used via `RSTRING_PTR` within rb_str_catf, which may allocate
and thus potentially trigger GC. Although `name` is still referenced
by a local variable, the compiler might optimize away the reference
before the GC sees it, especially under aggressive optimization or when
debugging tools like ASAN are used.

This patch adds an explicit `RB_GC_GUARD` to ensure `name` is kept alive
until after the last use.

While it's not certain this is the root cause of the following observed
use-after-poison ASAN error, I believe this fix is indeed needed and
hopefully a likely candidate for preventing the error.

```
==1960369==ERROR: AddressSanitizer: use-after-poison on address 0x7ec6a00f1d88 at pc 0x5fb5bcafcf2e bp 0x7ffcc1178cb0 sp 0x7ffcc1178470
READ of size 61 at 0x7ec6a00f1d88 thread T0
    #0 0x5fb5bcafcf2d in __asan_memcpy (/tmp/ruby/build/trunk_asan/ruby+0x204f2d) (BuildId: 6d92c84a27b87cfd253c38eeb552593f215ffb3d)
    #1 0x5fb5bcde1fa5 in memcpy /usr/include/x86_64-linux-gnu/bits/string_fortified.h:29:10
    #2 0x5fb5bcde1fa5 in ruby_nonempty_memcpy /tmp/ruby/src/trunk_asan/include/ruby/internal/memory.h:758:16
    #3 0x5fb5bcde1fa5 in ruby__sfvwrite /tmp/ruby/src/trunk_asan/sprintf.c:1083:9
    #4 0x5fb5bcde1521 in BSD__sprint /tmp/ruby/src/trunk_asan/vsnprintf.c:318:8
    #5 0x5fb5bcde0fbc in BSD_vfprintf /tmp/ruby/src/trunk_asan/vsnprintf.c:1215:3
    #6 0x5fb5bcdde4b1 in ruby_vsprintf0 /tmp/ruby/src/trunk_asan/sprintf.c:1164:5
    #7 0x5fb5bcddd648 in rb_str_vcatf /tmp/ruby/src/trunk_asan/sprintf.c🔢5
    #8 0x5fb5bcddd648 in rb_str_catf /tmp/ruby/src/trunk_asan/sprintf.c:1245:11
    #9 0x5fb5bcf97c67 in location_format /tmp/ruby/src/trunk_asan/vm_backtrace.c:462:9
    #10 0x5fb5bcf97c67 in location_to_str /tmp/ruby/src/trunk_asan/vm_backtrace.c:493:12
    #11 0x5fb5bcf90a37 in location_to_str_dmyarg /tmp/ruby/src/trunk_asan/vm_backtrace.c:795:12
    #12 0x5fb5bcf90a37 in backtrace_collect /tmp/ruby/src/trunk_asan/vm_backtrace.c:786:28
    #13 0x5fb5bcf90a37 in backtrace_to_str_ary /tmp/ruby/src/trunk_asan/vm_backtrace.c:804:9
    #14 0x5fb5bcf90a37 in rb_backtrace_to_str_ary /tmp/ruby/src/trunk_asan/vm_backtrace.c:816:9
    #15 0x5fb5bd335b25 in exc_backtrace /tmp/ruby/src/trunk_asan/error.c:1904:15
    #16 0x5fb5bd335b25 in rb_get_backtrace /tmp/ruby/src/trunk_asan/error.c:1924:16
```
https://ci.rvm.jp/results/trunk_asan@ruby-sp1/5810304

vm_backtrace.c

@@ -461,6 +461,7 @@ location_format(VALUE file, int lineno, VALUE name)
     else {
         rb_str_catf(s, "'%s'", RSTRING_PTR(name));
     }
+    RB_GC_GUARD(name);
     return s;
 }