Handle null pointer when attempting to build an error string

It's possible that libproj will return a null pointer instead of an
actual error string. In this case we now return a specific error
indicating this.

Fixes georust#73

src/proj.rs

@@ -94,6 +94,8 @@ pub enum ProjError {
     HeaderConversion(#[from] reqwest::header::ToStrError),
     #[error("A {0} error occurred for url {1} after {2} retries")]
     DownloadError(String, String, u8),
+    #[error("Got a null pointer while attempting to build an error String")]
+    NullPointer
 }
 
 /// The bounding box of an area of use
@@ -125,7 +127,9 @@ impl Area {
 
 /// Easily get a String from the external library
 pub(crate) unsafe fn _string(raw_ptr: *const c_char) -> Result<String, ProjError> {
-    assert!(!raw_ptr.is_null());
+    if raw_ptr.is_null() {
+        return Err(ProjError::NullPointer);
+    }
     let c_str = CStr::from_ptr(raw_ptr);
     Ok(str::from_utf8(c_str.to_bytes())?.to_string())
 }