[gdb] Improve early exits for env var in debuginfod-support.c

[gdb] Improve early exits for env var in debuginfod-support.c

There's an early exit in libdebuginfod's debuginfod_query_server, which checks
both for:
- getenv (DEBUGINFOD_URLS_ENV_VAR) == NULL, and
- (getenv (DEBUGINFOD_URLS_ENV_VAR))[0] == '\0'.

In debuginfod_source_query and debuginfod_debuginfo_query (which both
end up calling debuginfod_query_server) there are also early exits checking
the same env var, but those just check for NULL.

Make the early exit tests in debuginfod-support.c match those in
libdebuginfod.

gdb/ChangeLog:

2020-11-18  Tom de Vries  <tdevries@suse.de>

	* debuginfod-support.c (debuginfod_source_query)
	(debuginfod_debuginfo_query): Also do early exit if
	"(getenv (DEBUGINFOD_URLS_ENV_VAR))[0] == '\0'".