Avoid zero-length calloc (#3280)

Using `calloc` function with a count of zero elements is implementation
defined and it may cause allocation errors on some platforms (ref #3277).
This commit fixes `jv_mem_calloc` callers to avoid undefined behavior.

tests/base64.test

@@ -1,6 +1,10 @@
 # Tests are groups of three lines: program, input, expected output
 # Blank lines and lines starting with # are ignored
 
+@base64
+""
+""
+
 @base64
 "<>&'\"\t"
 "PD4mJyIJ"
@@ -15,6 +19,10 @@
 "foóbar\n"
 "Zm/Ds2Jhcgo="
 
+@base64d
+""
+""
+
 @base64d
 "Zm/Ds2Jhcgo="
 "foóbar\n"