Fix python pmParseInterval failure segfault

The following testcase segfaults because errmsg is NULL:
"""
import pcp.pmapi
context = pcp.pmapi.pmContext(target='local:')
print(context.pmParseInterval(''))
"""

Program received signal SIGSEGV, Segmentation fault.
0x0000003d2e43182d in parseError (spec=spec@entry=0x3d2e4591c6 "", point=point@entry=0x3d2e4591c6 "", msg=msg@entry=0x3d2e45b166 "Null or empty specification", rslt=rslt@entry=0x0) at rtime.c:202
202         if ((*rslt = malloc(need)) == NULL)
(gdb) bt

Make pmParseInterval parameters the same as pmParseMetricSpec()