Don't skip over symbol at start of file in _populate_symbols

Lets say we have an ELF with the following symbols

```
Symbol table '.symtab' contains 5 entries:
   Num:    Value          Size Type    Bind   Vis      Ndx Name
     0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
     1: 0000000000000035     0 NOTYPE  LOCAL  DEFAULT    2 aaaa
     2: 0000000000000022     0 NOTYPE  LOCAL  DEFAULT    2 bbbb
     3: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT    2 cccc
     4: 0000000000000054     0 NOTYPE  GLOBAL DEFAULT    2 dddd
```

Then pwnlib's ELF(binary).symbols will be `{'aaaa': 53, 'bbbb': 34, 'dddd': 84}`.
This is missing the symbol `cccc`, because it's value is 0.

This change checks the name instead of the value, because the value can
be 0 if the symbol points to the beginning.

The new and correct value of pwnlib's ELF(binary).symbols will be
`{'aaaa': 53, 'cccc': 0, 'bbbb': 34, 'dddd': 84}`.

pwnlib/elf/elf.py

@@ -908,10 +908,9 @@ def _populate_symbols(self):
                 continue
 
             for symbol in _iter_symbols(section):
-                value = symbol.entry.st_value
-                if not value:
+                if not symbol.name:
                     continue
-                self.symbols[symbol.name] = value
+                self.symbols[symbol.name] = symbol.entry.st_value
 
     def _populate_synthetic_symbols(self):
         """Adds symbols from the GOT and PLT to the symbols dictionary.