[llvm-nm] Remove redundant check for file validity.

We already perform it at the beginning of the function so we can't
arrive here with an invalid object. Also, add a test so that bugs
won't sneak in the future.

llvm-svn: 258982

llvm/test/tools/llvm-nm/invalid-input.test

@@ -0,0 +1,2 @@
+RUN: not llvm-nm %s 2>&1 | FileCheck %s -check-prefix=UNKNOWN_FILE_TYPE
+UNKNOWN_FILE_TYPE: {{.*}}invalid-input.test: The file was not recognized as a valid object file

llvm/tools/llvm-nm/llvm-nm.cpp

@@ -1240,9 +1240,7 @@ static void dumpSymbolNamesFromFile(std::string &Filename) {
     if (!checkMachOAndArchFlags(O, Filename))
       return;
     dumpSymbolNamesFromObject(*O, true);
-    return;
   }
-  error("unrecognizable file type", Filename);
 }
 
 int main(int argc, char **argv) {