symbols.c: avoid warn_unused_result build failure on fgets()

symbols.c: avoid warn_unused_result build failure on fgets()

In commit:

d37d63d symbols: prefix static symbols with their source file names

An unchecked fgets was added. This causes a compile error at least
on ubuntu utopic:

symbols.c: In function 'read_symbol':
symbols.c:181:3: error: ignoring return value of 'fgets', declared with
attribute warn_unused_result [-Werror=unused-result]
   fgets(str, 500, in); /* discard rest of line */
   ^

Paper over the warning by checking the return value in the if statement.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
Acked-by: Jan Beulich <jbeulich@suse.com>