Certain functions in glibc have "nonnull" attributes on pointer
parameters (even in cases where passing a null pointer should be handled
correctly). There are a few cases of such calls in flang: memcmp and
memcpy with the length parameter set to 0.
Avoid passing a null pointer to these functions, since the conflict with
the nonnull attribute could cause an undefined behavior.
This was detected by the undefined behavior sanitizer.
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