getdeps: dyndeps: gracefully handle empty files

Summary:
Don't error out if we can't read the ELF/MACH-O header; just treat
it is not an object.

Reviewed By: chadaustin, simpkins

Differential Revision: D19253434

fbshipit-source-id: c5ecc7f0bc7a20e2611b7e2ff754355155f095da

build/fbcode_builder/getdeps/dyndeps.py

@@ -251,7 +251,10 @@ class MachDeps(DepBase):
         with open(objfile, "rb") as f:
             # mach stores the magic number in native endianness,
             # so unpack as native here and compare
-            magic = unpack("I", f.read(4))[0]
+            header = f.read(4)
+            if len(header) != 4:
+                return False
+            magic = unpack("I", header)[0]
             return magic == MACH_MAGIC
 
     def list_dynamic_deps(self, objfile):