Slightly optimize h2_mplx_c1_streams_do(). #513

Job	Run time
Empty APLOGNO() test	43s
Default	4m 15s
All-static modules	5m 11s
Prefork MPM, all-modules (except cgid)	5m 56s
Worker MPM, all-modules	4m 32s
Shared MPMs, all-modules	4m 37s
Event MPM, all-modules, mod_cgid only	4m 55s
Event MPM, all-modules, no CMSG_DATA	4m 44s
Default, all-modules + install	5m 57s
Default, all-modules, random test order	5m 25s
GCC 10 maintainer-mode w/-Werror, install + VPATH	3m 57s
All-modules, APR 1.7.4, APR-util 1.6.3	5m 35s
APR 1.8.x, APR-util 1.7.x	7m 19s
Pool-debug	8m 31s
Shared MPMs (event), pool-debug, SSL/TLS variants	17m 58s
Shared MPMs (worker), pool-debug, SSL/TLS variants	21m 34s
Shared MPMs (prefork), pool-debug, SSL/TLS variants	22m 38s
litmus WebDAV tests	2m 32s
Regenerate ap_expr	4m 29s
APR 1.7.4, APR-util 1.6.3, LDAP	5m 8s
APR trunk thread debugging	5m 6s
UBSan	7m 23s
ASan	8m 48s
ASan, pool-debug	8m 2s
HTTP/2 test suite	14m 50s
MOD_TLS test suite	5m 59s
	3h 16m 4s

Provide feedback