trac#386: Remove overabundant and wrong declaration.

And replace it with unnecessary but right declaration.

I could not verify that we ever had oval_string_map declared
correctly.
trac#387: Fix segfault on incorrect content

Addressing:
  #0  0x000000391c035c39 in raise () from /lib64/libc.so.6
  #1  0x000000391c037348 in abort () from /lib64/libc.so.6
  #2  0x00007ffff7d390fa in oval_string_map_get_value (map=0x62a080, key=0x0)
      at oval_string_map.c:237
  #3  0x00007ffff7d057eb in oval_definition_model_get_test (model=0x61ec20, key=0x0)
      at oval_defModel.c:262
  #4  0x00007ffff7d05ddd in oval_definition_model_get_new_test (model=0x61ec20, id=0x0)
      at oval_defModel.c:443
  #5  0x00007ffff7d20cc3 in oval_test_parse_tag (reader=0x61eac0, context=0x7fffffffd840,
    usr=0x0) at oval_test.c:365
  #6  0x00007ffff7d15134 in oval_parser_parse_tag (reader=0x61eac0, context=0x7fffffffd840,
      tag_parser=0x7ffff7d20c63 <oval_test_parse_tag>, user=0x0) at oval_parser.c:59
  #7  0x00007ffff7d15608 in oval_definition_model_parse (reader=0x61eac0,
      context=0x7fffffffd840) at oval_parser.c:177
  #8  0x00007ffff7d0573b in oval_definition_model_merge (model=0x61ec20,
      file=0x61c9f0 "/tmp/1.xml") at oval_defModel.c:245
  #9  0x00007ffff7d055ea in oval_definition_model_import (file=0x61c9f0 "/tmp/1.xml")
      at oval_defModel.c:214
  #10 0x0000000000409c8c in app_evaluate_oval (action=0x7fffffffd990) at oscap-oval.c:390
  #11 0x0000000000407d21 in oscap_module_call (action=0x7fffffffd990) at oscap-tool.c:261
  #12 0x000000000040819e in oscap_module_process (module=0x616f00 <OVAL_EVAL>, argc=5,
      argv=0x7fffffffdc08) at oscap-tool.c:346
  #13 0x0000000000406b57 in main (argc=5, argv=0x7fffffffdc08) at oscap.c:79