stacktrace: prevent OOB-error in sysimage lookup (#30369)

Previously, with a multi-versioned system image, there might be additional entries at the end of the clone list
that do not correspond to an actual method (such as jlplt thunks).

Also some code cleanup for clarity.

fix #28648

src/debuginfo.cpp

@@ -1087,7 +1087,8 @@ static int jl_getDylibFunctionInfo(jl_frame_t **frames, size_t pointer, int skip
         for (size_t i = 0; i < sysimg_fptrs.nclones; i++) {
             if (diff == sysimg_fptrs.clone_offsets[i]) {
                 uint32_t idx = sysimg_fptrs.clone_idxs[i] & jl_sysimg_val_mask;
-                frame0->linfo = sysimg_fvars_linfo[idx];
+                if (idx < sysimg_fvars_n) // items after this were cloned but not referenced directly by a method (such as our ccall PLT thunks)
+                    frame0->linfo = sysimg_fvars_linfo[idx];
                 break;
             }
         }

src/staticdata.c

@@ -1016,6 +1016,7 @@ static void jl_update_all_fptrs(jl_serializer_state *s)
     for (i = 0; i < sysimg_fvars_max; i++) {
         uintptr_t val = (uintptr_t)&linfos[i];
         uint32_t offset = load_uint32(&val);
+        linfos[i] = NULL;
         if (offset != 0) {
             int specfunc = 1;
             if (offset & ((uintptr_t)1 << (8 * sizeof(uint32_t) - 1))) {

stdlib/Profile/src/Profile.jl

@@ -514,6 +514,7 @@ function tree!(root::StackFrameTree{T}, all::Vector{UInt64}, lidict::Union{LineI
                 # jump forward to the end of the inlining chain
                 # avoiding an extra (slow) lookup of `ip` in `lidict`
                 # and an extra chain of them in `down`
+                # note that we may even have this === parent (if we're ignoring this frame ip)
                 this = builder_value[fastkey]
                 let this = this
                     while this !== parent
@@ -532,8 +533,7 @@ function tree!(root::StackFrameTree{T}, all::Vector{UInt64}, lidict::Union{LineI
                 frame = (frames isa Vector ? frames[i] : frames)
                 !C && frame.from_c && continue
                 key = (T === UInt64 ? ip : frame)
-                down = parent.down
-                this = get!(down, key) do
+                this = get!(parent.down, key) do
                     return StackFrameTree{T}()
                 end
                 this.frame = frame