Check for backtrace_fd returning NULL

src/backtrace.c

@@ -35,16 +35,18 @@ void uc_backtrace_buf(char *buf, size_t len)
     buf[0] = 0;
 
     lines = backtrace_symbols(buffer, ptrs);
-    for (i = 1; i < ptrs; i++) {
+    if (lines != NULL) {
+        for (i = 1; i < ptrs; i++) {
 
-        int rc = snprintf(buf, remaining, "%s\n", lines[i]);
-        
-        if (rc <= 0 || (size_t)rc >= remaining) {
-            break;
+            int rc = snprintf(buf, remaining, "%s\n", lines[i]);
+            
+            if (rc <= 0 || (size_t)rc >= remaining) {
+                break;
+            }
+
+            remaining -= rc;
+            buf += rc;
         }
-
-        remaining -= rc;
-        buf += rc;
     }
 
     free(lines);