[Fix] `nvm_get_mirror`: disallow some non-URL characters #1128

Job	Run time
shellcheck_matrix (bash, nvm.sh)	47s
shellcheck_matrix (sh, nvm.sh)	1m 18s
shellcheck_matrix (dash, nvm.sh)	32s
shellcheck_matrix (ksh, nvm.sh)	47s
shellcheck_matrix (bash, install.sh)	34s
shellcheck_matrix (bash, bash_completion)	34s
shellcheck_matrix (bash, nvm-exec)	29s
shellcheck	8s
	5m 9s

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