Use std::string in mdebugread.c

Use std::string in mdebugread.c

This changes a couple of spots in mdebugread to use std::string rather
than manual management.  This is simpler, and also avoids shadowing by
renaming the variable in question.

gdb/ChangeLog
2018-10-04  Tom Tromey  <tom@tromey.com>

	* mdebugread.c (parse_partial_symbols): Use std::string.