Fix missing exe (start==vma) and update requirements

pwndbg/symbol.py

@@ -82,6 +82,10 @@ def add_main_exe_to_symbols():
         return
 
     exe  = pwndbg.elf.exe()
+
+    if not exe:
+        return
+
     addr = exe.address
 
     if not addr:

requirements.txt

@@ -1,2 +1,3 @@
 capstone>=4.0
-pycparser
+pycparser
+psutil>=3.1.0