[LLVM][Support] check for error return from dladdr. (#138369)

In case of an error, the DL_info struct may have been left
uninitialized, so it is not safe to use its members.

In one error case, initialize dli_sname to nullptr explicitly, so that
the later check against nullptr is guaranteed to be safe.

llvm/lib/Support/Unix/Signals.inc

diff --git a/llvm/lib/Support/Unix/Signals.inc b/llvm/lib/Support/Unix/Signals.inc
index 691e101..6668a29 100644
--- a/llvm/lib/Support/Unix/Signals.inc
+++ b/llvm/lib/Support/Unix/Signals.inc
@@ -826,14 +826,17 @@
   int width = 0;
   for (int i = 0; i < depth; ++i) {
     Dl_info dlinfo;
-    dladdr(StackTrace[i], &dlinfo);
-    const char *name = strrchr(dlinfo.dli_fname, '/');
-
     int nwidth;
-    if (!name)
-      nwidth = strlen(dlinfo.dli_fname);
-    else
-      nwidth = strlen(name) - 1;
+    if (dladdr(StackTrace[i], &dlinfo) == 0) {
+      nwidth = 7; // "(error)"
+    } else {
+      const char *name = strrchr(dlinfo.dli_fname, '/');
+
+      if (!name)
+        nwidth = strlen(dlinfo.dli_fname);
+      else
+        nwidth = strlen(name) - 1;
+    }
 
     if (nwidth > width)
       width = nwidth;
@@ -841,15 +844,20 @@
 
   for (int i = 0; i < depth; ++i) {
     Dl_info dlinfo;
-    dladdr(StackTrace[i], &dlinfo);
 
     OS << format("%-2d", i);
 
-    const char *name = strrchr(dlinfo.dli_fname, '/');
-    if (!name)
-      OS << format(" %-*s", width, static_cast<const char *>(dlinfo.dli_fname));
-    else
-      OS << format(" %-*s", width, name + 1);
+    if (dladdr(StackTrace[i], &dlinfo) == 0) {
+      OS << format(" %-*s", width, static_cast<const char *>("(error)"));
+      dlinfo.dli_sname = nullptr;
+    } else {
+      const char *name = strrchr(dlinfo.dli_fname, '/');
+      if (!name)
+        OS << format(" %-*s", width,
+                     static_cast<const char *>(dlinfo.dli_fname));
+      else
+        OS << format(" %-*s", width, name + 1);
+    }
 
     OS << format(" %#0*lx", (int)(sizeof(void *) * 2) + 2,
                  (unsigned long)StackTrace[i]);