[Fix] `nvm_get_mirror`: ensure only a valid URL is allowed #1147

Job	Run time
shellcheck_matrix (bash, nvm.sh)	41s
shellcheck_matrix (sh, nvm.sh)	46s
shellcheck_matrix (dash, nvm.sh)	39s
shellcheck_matrix (ksh, nvm.sh)	43s
shellcheck_matrix (bash, install.sh)	20s
shellcheck_matrix (bash, bash_completion)	22s
shellcheck_matrix (bash, nvm-exec)	26s
shellcheck	8s
	4m 5s

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